Ah yes, Guitar Hero. I've played everything up to Guitar Hero: Metallica excluding DS titles. My personal favorite of what I've played so far is GH2, with GH:M coming as a close second. I think my preference is based on difficulty, though. I find that I play the more difficult songs more often than the easy ones.
On the topic of versions that you may like, if you have a PS2, you may be able to find Guitar Hero Encore: Rocks the 80's somewhere. I loved that game, and really wish they had ported it to the xbox 360. The songs on it were catchy and really fun to play. You may also be interested in Guitar Hero 3. that one had the largest variety, with songs ranging from Slow Ride to Raining Blood. The number of lighter songs far outnumbers the number of heavy songs, which is good if you aren't a fan of heavy stuff. I think there's maybe five or six songs in the career setlist which I consider to be heavy, and of those, "One" by Metallica and "Number of the Beast" by Iron Maiden are included.
If you're picky with music, I would suggest checking out the setlist of the games before buying one. Scorehero.com has a list of songs in every game released so far, but the last time I checked, they didn't list the artist's name with the song. It should be fairly easy to find one on Google, though.
All this talk about Guitar Hero makes me want to play, and my friend hasn't returned my xbox to me yet T_T.