GURPS and Savage Worlds.
GURPS for the incredible number of genre books and flexibility, resources statted for it, and the realism allowed. It also helps that I've been playing/running it for over seven years.
Savage Worlds for sheer speed and Over-the-Top capacity allowed for with the Action Deck, Adventure Deck and the "Bennies".
I have friends that run d20 and I'm not a big fan of that system. I like a system that natively uses dice that allow for a curve.
I ran SW and SAS/TriStat at last year's Norwescon here in Seattle and people loved them both. I don't count SAS/TriStat as a favorite as I've recently had a part of the rules broken off in my ass. Admittedly it's my fault as I allowed players to over-use the "Dynamic Powers" power. I suppose that barring stupidity and ignorance on the part of the GM it's a great system as well.

this is covered in a thread already on this tribe:

analogrpgers.tribe.net/thread...c76bf9a

clicking on the link above will reveal the answers already put to this question.