It's hard for me to pick only one powerset out of all of them which is my favorite, but I think Willpower is really close to that number one spot. It was, and still is, one of the best defensive(Is that you call powersets like Willpower?) power sets, I must have made half a dozen characters with Willpower.
As for offensive powersets:
Melee DPS: Kinetic Melee; mainly because I thought it was one of the best looking powersets out there. I think I used to dislike the sound of it however, but unfortunately it has been too long and I have forgotten what it sounded like.
Ranged DPS: Dual Pistols, definitely. It had incredible animations, even though they sometimes were a bit long(I believe that they were shortening them a bit for better DPS in I24). The damage was good in my opinion. I can't really recall how good unfortunately. If the game comes back, I'll be glad to finally remember and relive all the things I've forgotten.
Crowd Control: It is a tough choice between Mind and Illusion control. I liked them both almost equally, but I think Illusion would be my favorite crowd control set because of the pets. Phantom Army is, and most likely always will be, one of my favorite powers of CoH.
Support: I never really played support characters, I stuck to tanks mainly and dps characters, but I have always appreciated players who used Empathy. It was such a great powerset, and the people who used it usually did their best to keep the entire team alive.