David Wise - Stickerbrush Symphony. I love Nintendo's usage of leitmotivs at their games. You will find this version in Donkey Kong Country 2, and recurrently over the saga. I am in absolute love with this song. Listen it on your own risk, as it's highly likely you will fall in love with it as well.
Motoi Sakuraba - Broken Balance. This piece found at Eternal Sonata is easily one of the best songs ever made for a videogame.
Jack Wall - Suicide Mission. Even if Mass Effect 2 may be my less favorite of the series (I love the first one), this piece while Shepard rallies their allies is pure gold.
Poets of the Fall - The Poet and the Muse. Alan Wake at its purest. It's here as a tribute to the consistence of its theme rather than the value of the song.
Akira Yamaoka - Theme of Laura. Silent Hill 2 is one of the best games I've ever played in terms of Sound Design. And probably the best Horror game.
Akira Yamaoka - Intro Silent Hill 1. The awesome mandolin is cool and unique enough to recognize Silent Hill 1 just by listening to this Intro.
Kōhei Tanaka - The One Who Hates Man. I love how magnificently the violins push tension up on this song found at the masterpiece Alundra.
Kōhei Tanaka - Requiem. Found at Legend of Legaia and Alundra. So touching.
Kan Gao - For River ...billions of lighthouses...stuck at the far end of the sky
Honorable Mentions:
Sojourner's Truth (Virginia OST - Lyndon Holland)
The Endless Dream (Dawn of Mana)
The Village of Inoa (Alundra)
Nightmare (Alundra)
Never Forget (Halo 3)
The Price of Freedom (FFVII: Crisis Core)
A Flower Blooming in the Slums (FFVII: Crisis Core)
071: Undertale (Undertale)
Green Hill Zone (Sonic the Hedgehog)
Ice Cap Zone Act 1 (Sonic the Hedgehog 3)
Knitty Knotty Windmill (Yoshi's Woolly World)
Irate Eight (Donkey Kong Country: Tropical Freeze)
Battle Against Giant Mutant (Tomb Raider)
Duel (Metal Gear Solid)
Boss Theme (Sonic the Hedgehog 2)
The Toxic Avenger - My Only Chance (Furi)
Waveshaper - Wisdom of Rage (Furi)
Danger - 7:53 (Furi)
Time's Scar (Chrono Cross)
Main Theme (Metal Gear Solid 3)
Main Theme (Halo)
The Opened Way (Shadow of the Colossus)
Sunlit Earth (Shadow of the Colossus)
Destiny (Medieval 2: Total War)
Baba Yetu (Civilization IV)
World 2 - The Hospital (Super Meat Boy)
Chainsaw Hedge Mayhem (Zombies Ate My Neighbors)
Overworld (Super Mario World)
Aaaaaaand of course..
Portal - Still Alive. "A great game is a great game. But a great game with an awesome theme song: That's gonna stick with people".