Colonizer would always be the most powerful as colonizing is what will ultimately allow you win a war of attrition.  If I have seven planets and you have five, I'll eventually win unless you're significantly better than me.

EDIT: of course, I suppose ultra-hard AI's already have all the resources they need so idk..

Not to be overly pessimistic, but I find the fortifier AI to simply be the best for any situation. Aggressive is... hostile with reckless abandon, bordering on insane (which, arguably, might fit rather well), and researcher is a push over. I haven't played around with economist... well... any, I think, but it's supposed to be in the same boat as researcher. Fortifier plays rather conservatively, and isn't much for throwing units away. Only downside to fortifier is their damnable mine spam.

Of course, TEC rebel with Pirate Mercenaries makes the suicidal attacks less insane and more tactical, but still. Haven't played the TEC much since the 0.75 changes, not sure how much different it has on the AI.