I really like music, and I really like games, therefore by logical extension I should really like music games. And I do! Starting from the top:
Dance Dance Revolution - Probably the first music game I played since "Simon" (yes, I'm dating myself here) and certainly one of the more popular ones, my initial experience was dulled somewhat by a crappy 3rd party pad and my big feet. I currently own Supernova for the PS2 and its quite fun, though currently living on the second floor limits my stomping around. I'd have to rate it my least favorite of the games on this list mainly because of the J-pop heavy soundtrack, but its fun.
Elite Beat Agents - Touch screen + highly stylized comic-book graphics + rhythm-based gameplay + wacky stories that could only come from Japan = FUN! The gameplay is solid, and the soundtrack is a nice mix of classic and current tunes from the US pop charts. If I have anything bad to say it, its that the audio quality of the music could be better. If a sequel ever comes down the pipe, it'll be a "Day 1" game for me.
Karaoke Revolution - You don't need Simon Cowell to find out that you can't sing, though Virtual Simon is available in the American Idol edition of the game. While this is more of a jazzed-up karaoke machine than a game, its still loads of fun. Check your dignity at the door with this one, which of course makes it a great party game.
Guitar Hero - I have been a fan ever since I played the first game at Circuit City. Admittedly, the game would be quite the same without the awesome controller, which takes it all the way to 11. The soundtrack, for the most part, rocks and with online play now added, the game only gets better. The downside is that non-rockers need not apply...or is that a plus?
Whether its stepping, tapping, singing, or rocking, music games will always have a place in this gamer's systems.