Sorry, I'm definitely a bit late to this! As others have said GH2 is the one i find best for the actual note mapping, the hammer-ons and pull-offs are the least forgiving (i.e. you've got to be pretty precise) but the actual fingering of the buttons is best I've seen.
Again, as others have mentioned, there are too many covers and some of them pretty poor when compared to the real tracks (especially those ones with distinct singers - the GnR cover as an example!) and the lack of full band participation is a bit of a swine, definitely lends a lot of support to buying the new Greatest Hits when it gets released as it basically looks to be an upgrade of GH2!
If you're choosing between GH3 and GHA then 3 definitely gets the nod, some of the tracks are a bit meh and the button mapping is a bit off in places but it's certainly a much better experience than GHA

Hope you enjoy it when it drops through the door
ps Fight through the hand-cramp