Bombs for Throwing at You, Wheatley's final boss theme. It's like "You Can't Escape, You Know", GLaDOS's final boss theme from the original, except it is faster, more frantic, and has some of Wheatley's Leitmotif mixed in at some points.
"Don't Do It". It's played twice, first during the confrontation with GLaDOS, and again at the analogous spot in the Final Boss battle. What's awesome is how it starts out calm, then grows in intensity at "Please press the Stalemate Resolution Button", and then goes freaking nuts after the Booby Trap goes off.
With the release of the first volume of the official soundtrack, we now have The Courtesy Call from the opening without the voice over. And it's as awesome as you imagined it would be.
Caroline Deleted. Even without knowing the context of the song, you can just feel that something bad is going to happen about half-way through... even though something good happens instead.
Your Precious Moon, the music that plays when Chell fires the Moon-portal. And yes, it is appropriately awesome.
Omg, What Has He Done? The first half of the song leaves you feeling terrified at how wrecked Aperture has become... and the second half pumps you up in preparation for one hell of a Final Boss battle that places the weight of all of Aperture on your shoulders.
The ending song, "Want You Gone", (spoilers, of course) is really good too.
There's something about the music box version of the turret opera that just drives you to tears.
And during the game you can find some of these turrets singing this.
"Wheatley's Song", by Miracle of Sound.
PotatOS Lament. This song only plays on the title screen while you're in the old Aperture Science testing spheres, but it's one of the most haunting, etherial melodies in the entire game. Despite the digital effects and the nigh-incomprehensable lyrics, the sheer emotion in GLaDOS' singing is enough to make you want to shed a tear.
"This Is Aperture", a Portal-themed take on "This Is Halloween". Some of the voices in there are so good you'd swear they were the original VAs. note This is made even more awesome by the fact that all of the voices were done by the same guy.
9999999 is the very first thing you hear upon playing Portal 2 for the first time. The first 1:07 are quiet and mysterious... and then, at 1:08, the full orchestra kicks in, letting the player know that, this time, the stakes are held even higher than they were the previous game.
Players only get to hear a small snippet of (defun botsbuildbots () (botsbuildbots)) on the main menu during GLaDOS's tests, but the full song truly shows how awesome it is. It recycles a single melody a few times, and each time it does so, it grows in strength until it peaks at 1:53. Combined with the visuals on the main menu of the turret assembly line, the song provides a perfect feeling of Aperture's machinery hard at work, and how it all combines together to form an impossibly large facility.