Games with good soundtracks

Dustforce has amazing soundtrack(tho uninteresting gameplay imo).

Mirrors edge, xenogears, Jet set radio future, Burnout Takedown 3, among others that have already been mentioned above like AC series

Anything with Jeremy Soule is mesmerizing, especially his pre-2011 work.

dust an elysian tail, S&S EP, stealth bastard, shoutout for Starbound even though it ain't out yet.

Bioshock has the best soundtrack ever

Well, here are some.

Jessica Curry: Dear Esther

I Have Begun My Ascent
Always (Sanford Mix)

Lifeformed: Dustforce

9-bit Expedition

Ben Prunty: FTL

MilkyWay (Explore)
Mantis (Battle)
Cosmos (Battle)

C418: Minecraft

Haggstrom
Sweden

Mike Morasky: Portal 2

Halls of Science 4

Markus 'Captain' Kaarlonen: Rochard

Poets of the Fall - Grinder's Blues
Markus Kaarlonen - Space Debris (Spacesynth Remix)

Module: Shatter

Kinetic Harvest
Amethyst Caverns

Jerry Martin: SimCity 4

Jerry Martin - Rush Hour
The Humble Brothers - Epicentre

Mike Morasky: Team Fortress 2

More Gun
MEDIC!

Edit:

Stop ninjaing me while writing long replies, damnit!

I'm surprised nobody mentioned the Papo & Yo Ost

Shatter, To the Moon, VVVVVV, Bit.Trip Runner, if we're sticking to Steam games.

final fantasy games, metal gear solid games, mass effect games, dragon age

The half-life games, particulary half-life 2, Civilization 4 and anything by paradox, the theme for Crusader Kings 2 still gives me the chills.

Morrowind, Icewind Dale, To The Moon, Halo series

Heroes of Might & Magic Series of course!

F-Zero X, Mariokart games, Pokemon games, Legend of Zelda games, Super Mario games, Super smash bros games. Nintendo rules.

Also, kingdom hearts games and final fantasy games have nice soundtracks too.

Mega Man X, Zelda Games

Space Rangers !!!

Mafia 1, Deus Ex Human Revolution, FTL, Mirror's Edge, Far Cry Blood Dragon, Dishonored, VVVVVV, Splinter Cell Double Agent, Trine 2, Terraria.

Muramasa: The Demon Blade, Chrono Trigger, Chrono Cross, Final Fantasy VI, Super Metroid, Metroid Prime 1-3, The World Ends With You, Super Mario Galaxy, Legend of Zelda (all excluding the CD-i ones), Double Dragon Neon (really 80s), Mighty Switch Force, Guilty Gear, Silent Hill 2, Shadow of the Colossus, Metal Gear Solid (all), Castlevania: SotN

Maybe I'll list a bit more later, I can go on and on with this

• Aquaria
• Sword & Sworcery
• Dustforce
• To the Moon
• VVVVVV

Quite enjoying the Saints Row 3 soundtrack!

Also really dug Shadowrun Returns, To The Moon, Portal 2, and of course HalfiLife 2

A lot of SNES games, like Yoshi's Island, Donkey Kong Country, Super Metroid, Super Mario Kart, Star Fox and obviously Chrono Trigger. Oh and this.
DuckTales' The Moon is pretty much the pinnacle of 8-bit music, but other Capcom games from this era had amazing soundtracks.
Castlevania: Symphony of the Night. Chills every time.
F-Zero games had great music.
What else... Silent Hill series has soem really climatic music.
The World Ends With You wouldn't be the same OST. I fit the game's atmosphere perfectly.
Heroes of Might & Magic 3 has amazing OST.
Megaman X
Street Fighter
Guilty Gear with one of the greatest VGM sountrack arrange album. Seriously, listen to it.
Super Mario Galaxy has one hell of an OST too.
Legend of Zelda games obviously. I don't even need to put the link, you know the theme I would to put here anyway.
Metal Gear Solid. With one of the most amazing intros I've ever seen.
Not to forget Super Smash Bros which compile the greatest OSTs ever.
Oh and of course Pokemon.

Should be enough. There are still many games that I would love to mention, but I don't want to make my post any longer.

You should give a look to 'Ilomilo' its an awesome Xbox Live game with amazing soundtracks,nice puzzles and more...Also you can make music on main menu like this

Bastion has my favorite game soundtrack. I also like: Deus ex, ftl, to the moon, VVVVVVV

Kirbys Dreamland

Donkey Kong Country 2.

Witcher 1, 2, Mass Effect 3