Guitar Hero II
Guitar Hero is still just as addicting as ever, as you find yourself continually saying "just one more song". It's still great fun to rock out and play some good tunes, although the setlist isn't nearly as good as the first game's. I hardly recognized any of the song names when I first read the list, although I still heard the majority of them before and just didn't know the name. The songs are all still fun to play and usually catchy, but it's just harder to get into a song and really feel cool if you don't actually know it well.
A couple definite improvements are the hammer-ons/pull-offs, which actually work this time and are necessary on the harder difficulties, and practice mode, which lets you work on and experience any part of any song without having to actually be good. Other improvements are minor, GHII is probably a better gameplay experience but it's just not as amazing when you've done basically the same thing before.