Okay, I'll be completely unbiased (I'm a moderator at tweakxp.com forums). I have all three on my system and use them for different purposes. I like tweakui because it's really the simplest to use, and I really like x-setup because it's the most comprehensive (it can be a little more complicated than the others but it does give pretty good descriptions of what the tweaks do). I like tweakXP for certain functions so I keep it as well (and it also gives pretty good explanations for most things). TweakXP is free for the first 50 uses, by the way, and the other two are completely free.