I played Guitar Hero 2 for a while, and when I switched over to Guitar Hero 3, I noticed a big change in the way it played and registered the hits. I could tell that you had a bigger time window to hit the notes. Guitar Hero 2 took more skill in my opinion, like you said.
I've never played Guitar Hero 1, and I don't think I want to. I've heard that you can't hammer-on and pull-off. That'd get on my nerves, I wouldn't be able to play that.
Guitar Hero 3 had some really neat songs. My favorite songs are Cult of Personality, Take This Life, Through the Fire and Flames(what? It's fun to play), Don't Hold Back, Raining Blood, FCP REMIX, and The Devil Went Down to Georgia. I think I'm missing a few.
I have yet to play Rockband. I'd love to play it, but I do not have the money to get all the crap. It'd be cool to be able to sing and play the guitar at the same time. You'd have to know the songs lyrics, I suppose. I hope there are some songs on there I recognize.
I voted for Guitar Hero 2 and Guitar Hero 3.