Cannabalt had an awesome soundtrack, but I think Skyrim's theme is one of the best audible experiences a game has ever delivered. It's the most epic song ever dedicated to the player.

I can't pick one OST, but I can tell you that Rareware always had spectacular music.

I still love the first Sin & Punishment OST. It has a cool synthesizer dance groove with a crazy electric guitar playing solos all over the place in every track.

My favorite song is still "Within Earshot." It has a off-kilter drum beat with jazzy piano chords and a wild electric guitar synth solo. I'm not even sure how composer Toshiya Yamanaka even thought of all the ornaments and guitar licks for that one. It fit surprisingly well with Saki's dream sequence of running through a Space Harrier world.

http://www.youtube.com/watch?v=m-RJASd2HMI

This'll always be a favorite - http://youtu.be/lIO1sajyx_o

Also, the Shining Force 2 soundtrack holds an immense amount of nostalgia for me - http://youtu.be/-NFcun0Nets