My favorites from Best to least
1.) Guitar Hero World Tour: Although the tracks aren't as challenging as GH3, the fact that you can finally play drums, vocals, bass, etc. Really created a lot of hours of gameplay.
2.) Guitar Hero 3: Best designed tracks IMO.
3.) Guitar Hero Metallica: Again full instruments across the board with "master of puppets" although the other artist tracks are kind of lame.
4.) Guitar Hero 2: Some more good tracks
5.) Guitar Hero Rocks the 80's: meh
6.) Guitar Hero 1: even more meh
7.) Guitar Hero: Aerosmith: By far the worst. Way too easy, there are only 2 songs on their worth playing.
When Smash Hits comes out in June....that will probably be the best one. *crosses fingers*