Unlike Nightbane -- where I made something ridiculously imba even before talents, spells, or even skills, HU2 requires quite a bit more effort to bust. There are some really powerful super abilities, and some good builds make strong use of them. No HU2 creature is as resistant to everything as Nightbanes are, and magic in particular is a gigantic weakness for superhero characters.
However, HU2 has a few major kinks. The biggest issue is the Alien and Mega-Hero templates and somewhat the Mutant template. While Mega-Hero is considered "optional rules" in HU2 for more powerful characters, Alien is not.
All HU2 superhuman builds have the option of megahero powers, and the best penalty is forced transformation. There's no limit on transformation time, making it essentially a pretty Nightbane Morphus. The obvious megachoice is immortality. Megaheroes automatically get supernatural PS, though in some cases it's a pretty low value. They get 1.5 times their normal SDC (or MDC), and magic has half duration unless the character has a magic vulnerability. We would never choose magic as a vulnerability, ever. Mega immortality makes you literally unkillable while mega-transformed, your basically has to be atomized to die. Nobody goes to that kind of lengths to kill someone. Megastrength is the second good megapower option and it basically gives you +20 +avg 7 PS, which can let some heroes rival Olug and most rival or surpass the cow.
Aliens can hit ANY power category in addition to their alien racials. This is actually totally okay, since any Alien template character could become some imba Rifts class (shifter, battle magus, techno-wizard, etc); likewise more absurdly powerful D-Bee races in Rifts could also become any of the power categories and that's fine. What Alien does is add power for free though, which is dangerous. It's especially significant because aliens can get base MDC by having either high gravity environment or tough hide/natural exoskeleton. High Gravity is the most cheatery environment ever, tripling SPD attribute, granting PS, and giving MDC. In addition, you can add in mineral bodytype for +180 MDC and more PS. Rhino gives more PS, Demon gives multiple arms (+1 apm). For most of my top tier builds, I'll be using high gravity mineral alien.
The general uber builds are:
Mutant/Experiment: 1 major + 3 minor
Mutant/Experiment: 2 major
Mutant/Experiment: 5 minor
Mutant/Experiment: 1 major + 1d4+1 minor psychic powers or 1 super psychic power (for TW item use)
Mutant/Experiment: 3 minor + 1d4 minor psychic powers
Mutant/Experiment: 2 minor + 1 super psychic power
All mutants take ambidextrous as mutant trait and unstable power level growth trait. Level growth could be replaced with the emergency alter physical structure trait, for when situations get really bad.
All experiments take forced transformation for the 1d4*10 SDC and 1d6+4 PS bonus. There are other, less good traits that don't increase size, most notably chemical resistance. No facial features is also okay but superhuman access to TW metamorph stuff is kinda limited, so it's a pretty big drawback.
A mystically bestowed character with mega-hero immortality can get 8 low level spells and 1 imbalanced major super ability. The 8 low level spells are inconsequential; the real advantage is that the hero can use TW items with his one imbalanced major. The one imbalanced major super ability in question is Multiple Beings/Selves. The multiple selves have mega immortality, making them nearly unkillable. He can make 1 clone per level of his experience, although the clones are one level lower than him. He has an average MDC in bestowed form of about 170 (plus physical skill bonuses), plus hit points of relatively normal. Each clone has minimum 20(s) PS, 18 PP, 24 SPD and avg 75+PE mana plus 7.5 avg per level. This character becomes retarded by about level 3 with 3 unkillable kage bunshins all with caster abilities and low-superhuman power levels. If he's teamed with a character who can grant energy spheres, this guy can provide the mana for any ritual ever. Megahero immortality makes this character imbalancedly powerful and because it's in the core as an optional rule (and the devs acknowledge that megahero makes you pretty imbalanced) this shouldn't be seen as a gigantic exploit. It is BY FAR the worst case scenario and it is only really bad when paired with Rifts-tier magic.
Another option for Mystically Bestowed is to start with basically any spell in the game. He can only pick a few but he can pick the more abusive ones like Energy Sphere, Talisman, and Create Scroll. With Create Scroll, he can create translatable scrolls of his rape spells to allied casters. He kind of tapers off in power once his allies have all his spells, but he does retain mega-immortality and can still use any TW devices with the same mana pool as the above character. You may want to give him mega-strength instead and kill him off so you can roll something that can actually learn spells. Obviously, you'd relearn all your high level rape from your allied wizards and pick something gay like chiang-ku shifter or battle magus. A similar build uses Enchanted Object but you are limited to spell levels 1-12; you get a few more good powers, lose some durability, and gain one unique special effect. The unique effect is generally pretty crappy, except for complete magic immunity. The problem with the immunity effect is that it permanently gimps your ability to get durability and is overall not a very good idea. I would go for Mystically Bestowed if I wanted to use scrollhax.
The problem with Mystically Bestowed as a category is that the character must do the bidding of whatever's granting him his powers. He's basically a priest, and that's cool for RP but he's gotta pick the right god. Kym-nark-mar ftw?!?
Enchanted Object gets to use scrollhax as above but is not quite as good. He can use TW though even with major super abilities, so there are some decent builds. I don't think any of them are "cow good" but there's always sonic speed for a mini-Harken build. Probably not as good as Mystically Bestowed kage bunshins though.
Enchanted Weapon is a character packing a rune weapon as his main ability and loses all of his megapowers if his sword is ever lost. This makes him kind of vulnerable right out of the gate. I would take mega strength rather than mega immortality; even if you don't die right away, someone just jacks your sword and you're hosed. Better to be more capable of avoiding death. They have around 53 MDC in transformed mode, plus normal HP. We'll assume they do 6d6 weapon damage.
Mystic Weapon of Order can get some ridiculous strength by doubling up on the strength weapon option plus megastrength. The best options after strength are the +6 damage bonus, fly, mystic shield, and invis superior at will. The +6 damage bonus applies to the wielder, so he can use it to wield a battle fury blade doublestriking at +6 damage per hit. Not too bad, considering this build will probably have over 70 PS.
Mystic Weapon of Chaos has a few more utility options but not as much strength. +6 damage, portal of fear (makes a grappling tentacle portal, really nice), invis superior at will, fly and mystic shield are the best overall options. There are some decent other weapon choices. In this case though, the +6 damage is probably better off used with guns and a Chaos Weapon wielder probably wants spells so he can power TW items. He's an inferior choice overall other than his +6 damage with guns. Megastrength doesn't do him much good, but it's better than nothing? I'm having trouble making a working build for this guy that is dramatically better than nightstalker mystic (which is kind of a benchmark).
For the Mystic Weapon characters, they either get 1 major or 3 minors, although they get a spells option it only allows spells up to 8 and doesn't allow learning new spells. This, obviously, is kind of mediocre and the base mana is really bad.
For the Weapon of Order character he probably wants Teleport (major) for perfect gap closing with his monster strength and melee damage. He could also go with 3 minors and get extraordinary speed and 2 other +APM minors, for +3 APM total and some good dodge/parry bonuses. He might also be able to work automatic dodge into a minor. Because he's a Weapon of Order wielder, Karmic Power (major) might also be good to completely screw his opponents (it nulls all enemy combat bonuses) but Karmic Power relies on being a good guy all the time. Obviously, Invulnerability works for anything. APS: Stone or Metal can also be used for more PS but less mobility.
Mutant and Experiment heroes (Mutant is really ideal) have some power flexibility.
One major + TW use is a big draw. This -needs- to be a durability increase as the mutant is not MDC by default. APS Liquid, APS Metal or APS Ice are the most flexible options but Invulnerability is always there. Alternatively, it can be something imba like Slow Motion Control or Karmic Power and he can rely on EBA, PBF, or TW mods to give him protection. He's still mega-immortal. However, the durability powers get bonuses from being a mega-hero (+50% MDC) and APS Ice and Liquid in particular have a lot of utility. The mutant character with the level growth unstable perk can get a major on level-up and combine another imbalanced power with his first one.
3 minor + TW use is inevitably going to be harder to break. Manipulate kinetic energy for double damage physical attacks is insane in team play. Otherwise, he pretty much can just rely on getting +3 APM and some form of mobility and some combat bonuses. 2 minor + a super psi is not really worth it compared to 1 major and a super psi. The best combo there is manipulate kinetic energy with super-TK for some really imbalanced item throwing, which might be cool.
1 major + 3 minor will give some mobility to a major. I really, really like wingless flight with APS Ice, but this stuff isn't really broken. It's nice though. Nowhere near as good as 1 major + TW + some random psi.
2 major is really sick because you can do some dumb combos, especially with the possibility that mutants give for level-up powers. Multiple Beings/Selves with APS of any kind is pretty retarded, Plasma being a huge offender for huge damage output and fly mobility. Unfortunately, 2 majors and no TW means combining Multiple Beings/Selves with any non-durability power is fucking dangerous, even with mega immortality.
So overall, the biggest abusers (unless stated, all use the basic Alien + Mega-Hero template: high gravity mineral alien, mega hero immortal transformation)
Build 1: Plasma/Electric Kage Bunshins
This build uses Demon multi-armed aliens rather than mineral aliens for +1 APM. Mineral adds more durability, though.
I choose Mutant with Ambidextrous/Experience Growth and 2 majors
Major super abilities: Multiple Beings/Selves, Alter Physical Structure: Plasma (or Electricity)
mostly normal stats (HP is mega) except
Triple normal SPD
+avg 7 PS (normal)
avg 142.5 MDC in normal form, +90 in plasma form (plus physical skills)
Can create 1 copy of self per level, copies are 1 level lower than creator.
Deals 10d6 damage to anything it touches (in plasma form)
Deals 20d4 damage to anything grappling it (cannot actually grapple since intangible)
Flies at 60mph +20/level
Can throw 10d6 +2/level plasma bolts with 200ft +20level range
Can throw 4d6 plasma bolts with 300ft +30/level range
Takes half damage from energy (in plasma form)
Immune to physical and heat (in plasma form)
Takes huge damage from cold, water, etc.
This build basically gives the character a bunch of flying, unkillable plasma allies that deal monstrous ranged damage. He starts off reasonably powerful and gets absurd at higher levels due to his per-level speed increase and massive number of clones. He also gets superpowers every 3 levels at 3/6/9/12/15. He does not have very good endgame scaling though, and is better off only keeping poorly-equipped clones in plasma form later on. He is easily ignored with i2e, but due to getting superpowers every 3 levels he may have some other means of damage by level 3 or 6. He can also dual wield rifle weapons, so against an i2e enemy he still has his superior speed and particle beam rifles or multiple grenade launchers. Due to having so many clones, he can arm some of them as machinegunners or other means of support.
Build 2: Slowing Kage Bunshins
Mutant, Ambidextrous/Experience Growth, 2 majors
Multiple Beings/Selves, Slow Motion Control (or Gravity Control)
avg +25 PS (35.5) normal
avg +382.5 MDC
Can do a no-save half-apm half-bonuses attack
This build is weird, since it relies almost entirely on mineral alien tank and normal weapons. It has to use mana cell weapons to even use TW, and it will never be able to use TW normally unless it spontaneously develops Mimic. Even then, it will have to Mimic a psionic ally or enemy and most likely, the clones will be dispersed doing various things. It loses pretty hard to the plasma alien mutant unless armed with cryo-weaponry of some kind. However, the character has the unique ability to royally dominate a battle if his kage bunshins are allowed to get close due to the slow-motion attacks. His clones are also incredibly hard to hit without concentrated fire. He can 1v1 almost anything except someone else also using kage bunshins. He is somewhat vulnerable to the cow due to his low magic save, but the cow cannot do anything but cast spells and has half APM with which to do so. The gravity control version works a little better by improving mobility of kage bunshins (multiplying a multiplied speed is kinda dumb) and completely destroying anyone with bonuses. He is only really vulnerable to a really powerful mage and some MB/S builds.
Build 3: Mage Army
Mystically Bestowed, Mega-Hero Immortal/Transformation, Multiple Beings/Selves, 8 spells 1-5
PE/PP minimum ~20 (thus good bonuses/saves)
PS avg +27.5(s) (roughly 38)
avg +550 MDC
avg75+PE+avg 7/level mana
This build has ridiculous tank and speed like any mineral high gravity alien, but he is pushed to physical perfection thanks to Mystically Bestowed. His army can't cast any truly epic spells, but 1-5 has enough good stuff that he can really do some damage. In core, Mental Blast is level 5, as is Domination, while Magic Net is level 4. Astral Projection is there too. This guy has quite a few decent tools early and is a really big problem late, since he has full access to TW gear and each clone has enough mana to end a fight. He is a counter to casters due to his highish saves and magical aptitude. The plasma army can beat him early but it's as easy as TW ice guns lategame. The cow has no chance of beating even two of him as long as both of them stay out of Darksong range. In very close combat, the cow still might not beat him because he is unlikely to stun 4 or 5 kage bunshins for more than a melee round. Slow army build probably loses as well, as their magic saves are low.
Build 4: All the Strength
This build replaces megahero immortal with megahero strength.
Mystic Weapon of Order, +6 damage, fly, mystic shield, invis at will, strength x2 + 1 major
Alter Physical Structure: Metal
+1610 MDC in metal form (-1200 in normal form, ohnoes still 410)
+avg 97(s) PS (avg 107.5 before physical skills)
Triple SPD, which is halved in metal form
Can turn invisible at will, fly at 50mph (half in metal), create a 200 MDC parrying shield, and deals +6 damage with all attacks.
This character hits so hard, the universe shudders whenever he does not pull his punches. He's also unkillable and can just straight-up trade blows with almost anyone. He has a decent magic save (not listed) but no ways to boost his PE for more. He's really imba with a team, but is not actually one of the "titans." I just thought it would be amusing to build a character with over 100 supernatural PS. Almost any of the MB/S builds above can beat him.