I’m a big fan of HERO. It’s so flexible, I’ve yet to find a character concept I couldn’t model to make it work the way I wanted it to.
I’ve tried others, like GURPS and the Mutants & Masterminds system, but there’s always some little wrinkle that leaves me unhappier than I would be with the results in HERO. (Though not so unhappy as I wouldn’t use those systems out of hand.)