My all-time favorite for any system is Classic Deadlands. Its creepy, campy, fun, and disturbing all at the same time.
For d20 stuff, Midnight is by far out in the front. Its the only world I have seen that encourages the PCs to be heroes, not for money, profit, or power, but for ideological and social reasons. Its a focused campaign setting, but still leaves a lot up to the DM so he has tons of wiggle room. Having played and run a good amount of Midnight now, it plays and runs unlike anything else out there.
I like Iron Kingdoms so far, but haven't had a chance to play it yet. Its a little too steampunk/mecha crazy for me, but I really like the world background and tone. I'm not likely to run it as-is, but I will probably steal liberally from it.
Testament is a great setting too. I've only played it a few times, but what we have done has been great, and GR did an incredible job of presenting a potentially touchy subject with class.
I am also looking forward to the Thieves' World stuff from GR coming soon. I have all the old Chaosium Theives' World stuff, and the setting is so rich and bizarre, you never know what to expect.
I like Dark Legacies from what I've seen so far, but with only one book out right now, its kind of hard to know what direction the world will take. I'll definitely be picking up the next book though.
Finally, I loved Grimm. We played it until our group of kids hit 5th level and escaped, but it was a blast. I'm REALLY psyched up for the big hardback version of Grimm FFG is releasing later this year.