One of my favorite, if not THE favorite gaming OST of mine is still the NeoTokyo OST, which was some random mod for UT, if I remember correctly:

It was also the first thing that got me into drum'n'bass a looooong time ago. I have so many favorite pieces in there, it's so awesome!

What I'm also missing in here are some of the WoW soundtracks. No matter what you think about the game/the addons themselves, but Blizzard did an outstanding job creating a fitting OST for every expansion:

The whole Pandaria add-on is based on adventure and discovery of a new culture (which is based on Eastern cultures irl). It then quickly turns into a conflict between the Alliance and the Horde, fighting for predominance in that new country, while you still have the people from that culture living there and suffering from the conflict. And just the opening of that OST tells that story without words, it's incredible. They did an outstanding job on all of them.

Other than that, +1 for the Super Meat Boy and Shovel Knight linked here in this thread for representing awesome chiptune music.