I've become somewhat of a modern music game junkie after giving Guitar Hero 3 a shot last Christmas. I've played them all save for Rocks the 80's (I refuse to pay 50.00 for a glorified expansion pack, and I am annoyed that Activision refuses to lower the price despite the game being out for over a year.) or Areosmith. (It's slightly cheaper but there are so many other games I'd rather buy first.)
Personal favorite out of all them has to be GH2 sure they're mostly covers but the ratio of fun/good songs is much higher compared to 1 or 3. Also the fact that 2 has Trogdor also balances the scales highly in it's favor.
Of course after hearing the news about Ozzy's involvement in World Tour (See article below.) and the inclusion of his solo hit Crazy Train (A song I've wanted to see in one of these games since the first GH back in 05.) we might have a new winner.
http://ps3.ign.com/articles/897/897355p1.html