Nothing makes gaming more fun than charging into battle with some awesome tunes blasting from your speakers. Or as the main hero falls in battle to a soft violin. Music and sound is just as big part of gaming as any other gameplay mechanic. Its the part that leaves a mark on your subconscious, to whenever you hear it, you can't but help be reminded of the first time you struck down a dragon or conquered worlds.
So what game is it that's had the best music for you? It could be just one song in the entire game that stood above the rest, or an entire soundtrack that lead you through lands.
For me? Two games. The first is one we all know, Metal Gear Solid. The strings and brass has made me shed many tears for our headbanded ol' ninja.
And the second game, a slightly less well known game, Afro Samurai. The sound track was pure gold in this game, and possibly the best thing about the game. I still liked the game, but I wouldn't have liked it nearly as much if it hadn't been the same music.
EDIT: also maybe throw in a comment as is to why it was so great to You.