I thought the coolest abilities were the evolved forms. My personal favorite is the Biological's Predator Form, especially with its 100 focus regeneration per turn.
If shadow blend counted, that would be my pick of choice, but since it doesn't I'd have to pick predator form, 100 focus back per turn means you can use destroy every other turn, that said, it has a really rediculously long cooldown.
I would have to vote for predator as well. I like it more than toxicant (just played bio so far) because the requisite skill is more worthwhile as well. Then there's that whole thing of 100 focus regen....and yeah that's pretty swank.
I don't really like any of them that much, tbh, mostly because none are maintainable or last through the whole battle, unless you're only facing one normal opponent. My breakdown:
Predator: Only good for a focus engine (the damage boost is almost too small to matter). As a bio you usually don't need that much focus (unless you use Destroy, but Destroy is bad). I usually end up with this as a filler just because I use Agile Exposure a lot.
Toxicant: One of the better forms in my opinion. The +40% damage is good for everybody, and if you're a poison bio the poison piercing doesn't hurt either. Also more maintainable than any other form.
Wraith: The speed boost is good and the damage boost is okay, but the focus part is mostly useless. Also, as an electric psycho, I have the ability mini nuke already, so skipping a turn for slightly better nukes might not be worth it.
Psychopath: One of the strongest forms, but more useful for shadow psychos, which I find slow and boring. The long cooldown also sucks.
Guardian: I don't need any more damage reduction, so it just gets in the way of my debuff casting.
Avenger: This is alright as it lets you skip Regulate, but it's damage bonus isn't that great.
