Conan
Hands down the best OGL game I've played. Not just the game but the supplemental material. The class splats are so good. I really can't say enough good things.
True20
Though I have not yet had the opportunity to play I have read revised cover to cover and I am really impressed. I think it may be my next go to since I'm a bit disenchanted with base D&D these days.
Microlite20
Honorable mention. I just think it's really neat. It reminds me of the old Middle-Earth game-books. Simple, elegant, a real delight.
There's plenty others. I liked Spycraft, particularly the Shadowforce Archer material. Arcana Evolved has some concepts in it that are beyond compare. The Thieves' World material from Green Ronin is great. I am sad that I never get to use so much of this material that I own. I guess that's the downfall of the gaming junkie though.