Lists.

First off: sorry for the hiatus. It will happen again.
Right to business then!

Entire websites have made their living off of lists, so I chose to make my own.

Some background first!
Modern gaming has evolved beyond mere gameplay (your phone apps don't count, sorry). More and more developers are striving to create believable worlds for the players to immerse themselves into. Great strides have been taken in recent years in the formation of atmosphere within games. While my personal opinion is that 2K's Bioshock series is at the forefront of this evolution in design, there are others who manage it very well. It is also my personal opinion that one of the most important things lending itself to the creation of this atmosphere is the music found within games. To that end, here is a list of the 5 video game tracks that have left the greatest impression on me.

-The Elder Scrolls: Morrowind - Main Title. This song underscores everything that was great about this game to me. Every time I hear it I am thrown back to the hundreds of hours I spent scouring that volcanic island to the darkest depths of it's strange reaches. It had the most soul and depth of any Elder Scrolls game yet, and as my first sandbox/rpg it was a life experience that will never leave me.
Morrowind - Main Theme

-Bioshock: Infinite - Elizabeth's Theme. The newest example from this list. It won't be the last Bioshock title you see on this list. These games are on such a higher level than what almost any other studio is doing right now. Their story is always intoxicating, their twisted atmosphere inescapable, and this song lays out the beauty that can be found in Infinite's message perfectly.
Bioshock - Elizabeth's Theme

-Halo: CE - Opening Suite. God, how many times did I start this game up? The weight that this game left on my shoulders every time I had finished it was so much heavier than you expect from average shooters. That said, the score that finds itself interwoven with this game is its true masterpiece. There are harsh drum beats during heavy action, somber piano strokes in the places of loss, and dreadful strings when all seems lost. Behind it all this theme binds the game together into one beautiful sense of desperation, of struggle, and a hope that refuses to give up.
Halo - Opening Suite

-Bioshock 2 - Pairbond. If there is one thing that the second game did better than the first, it was the soundtrack. Pairbond manages to capture a strange beauty with its other-worldly violin - both giving voice to the broken city beneath the waves, and serving as emphasis for the strange dynamics that it's inhabitants perpetuate.
Bioshock 2 - Pairbond (Main Theme)

-Assassin's Creed: Revelations - Main Theme. This is my favorite offering from the series. Despite its criticism, I feel that it did perfect justice to the end of the Ezio trilogy - important because of how perfectly they nailed his character. Ezio was both tragic victim and comedian, badass and gentleman, and this song accompanies his twilight tale to perfection. The blending tribute to Altair bleeds through the notes and the combination haunts me every time I choose to play it.
Assassin's Creed: Revelations - Main Theme

Notables that were close second: Anything from Genesis-era Sonic, Mass Effect, Dragon Age: Origins, The Song of Time, Tales of Symphonia, Windwaker.
                 -I have to also include Ain't No Rest for the Wicked by Cage the Elephant, found in the first Borderlands game. While it is only found in the start-up sequence, this song fits the awkward character of that game so perfectly that I will forever associate the two of them together.