I've played a lot of them, but I've learned that I prefer a game system that has been tooled to the specifics of the setting and theme of the game to be played.
GURPS, D6, FUDGE and Savage Worlds is on its way. Looking at FATE as well. I'll be interested to see how this poll turns out. I'm wondering if there is really any more room (or demand) for generic systems, no matter how clever they might be. It almost seems that it's more important that a new system be couched in a really compelling setting, and trying to "port" it to another setting should be an afterthought if it's well received.
GURPS is the only one I've tried, and it's my favorite system. I want to try Savage Worlds (hopefully soon!) and Hero (not until the release of 6E, is that this GenCon or next?), though, when I get the chance.
I voted D20 Modern and True20. Sorta inaccurate, though. I've played some d20 PbP, but I have never had a chance to actually play True20. I figure I spent the money, though, so I should at least get to vote for it.
Whoops. Posted in the non-poll version. I'll just copy/paste over here...
Yeah, Savage Worlds. It's my new 4E/go-to game platform. Love it.
Also Dogs in the Vineyard. Which does have an implied setting, but we used it as generic.
I already voted and posted, but forgot about one of my favorites... the "Legend" series from Dark City Games. Legends of Time and Space, Legends of the Ancient World and Legends of the Untamed West. A really great series of simple tactical rules for roleplaying and/or their pre-programmed solo adventures that pick up where Metagaming and The Fantasy Trip left off in the early 80s.
Besides GURPS (which I played and wrote for), Hero System, Unisystem, and FUDGE, I can also include Simply Roleplaying! by Microtactix. While all of them are good, I would have to rate GURPS as my favorite of the lot.