Alternity (specifically Dark*Matter) is probably the best i’ve ever played. Munchkin, Paranoia, All Flesh Must Be Eaten and Star Wars D20 are other big favorites. I’m actually participating in a D&D session currently (after not having played in years) and it’s still kinda fun. I enjoy more present day settings where geography/history isn’t such a burden and can be molded to fit a game. Anything sci-fi/paranormal does it for me as well.

I’ve played GURPS and, to be honest, it seems a bit broken in the sense of min/maxing, but I’m sure there’s a way to go about things to solve that problem.