I've enjoyed a number of games with mediocre soundtracks, but every one of my favorite games has an absolutely brilliant OST. For example, the Final Fantasy series.
Nobuo Uematsu's work does more to create a sense of atmosphere or convey emotion than just about any dialogue or graphics that I've seen in games. I don't wanna say the series would be bad without him, as Square has an amazing lineup of composers (Yoko Shimomura, Yasunori Mitsuda, etc.), but it certainly wouldn't have been the same.
Final Fantasy IX is by far my favorite video game OST. It's about as close to perfect as a soundtrack can be IMO.