1.  Chrono Trigger - Even for a 16 bit game, this is still THE soundtrack in my book. Every single song is memorable.  The soundtrack was ahead of its time 15 years ago, and frankly most game composers still haven't caught up.

2.  Castlevania: Symphony of the Night - A mix of eclectic jazz and rock with a few epic battle scores.  It's just awesome and does the amazing job of making you forget just how godawful the voice acting was.

3.  Little Big Planet  - So much good music that fits each level like a glove. You'd almost think the levels were designed with the music in mind.... hell they probably were.

4.  Katamari Damacy - Easily the weirdest and most quirky soundtrack ever made, and it's pure awesome. You cannot play the game without having the main theme song stuck in your heads for hours afterwards.

5. Grand Theft Auto: Vice City - Possibly one of the greatest 80's music compilations ever made. Not only that, but it completely elevates the game to another level and makes Vice City the best "period piece" game ever.