I had a tough time with this one. I’ve heard a lot of great music over the years and played a lot of different soundtracks to death. (Before YouTube and Pandora, it was safe to say that game music dominated my collection.) I love the classical sounds in games like Dragon Quest 8, but I also enjoy the metal sounds of the Twisted Metal series and Command & Conquer. I grew up with the electronic sounds in games like Megaman. That sound evolved into something I really enjoy in games like Shatter and Motorstorm: Apocalypse.
I think if I had to put anything at the top of my list, Final Fantasy XI would be my choice, mainly for the variety that exists. There are a lot of great pieces, both mellow and intense. (This opinion is only based on the game’s soundtrack up to Chains of Promathia, but I’m sure the later expansions have had some good music as well.)