Blow Me Away by Breaking Benjamin, used in Halo 2 at the climactic battle scene. After a gameful of sweeping orchestras and choirs, it decided to ROCK OUT.
That's the equally-impressive vocals version, but this is the version you hear as you're running the gauntlet of, oh, dozens of aliens doing their best to splatter each other into little chunks of goo.
The best part of that song is how it crops up as you enter the room to a bunch of elites and brutes fighting, and then launches into the insane rock as two hunters join the party. At that point, all ideas of hanging around until the groups destroy each other goes pretty much out the window. (And with that, we have the answer to how to stop the player sitting out of your huge battle scene - invoke Autobots, Rock Out!.)
The version including the vocals was also used as the theme tune for Major League Gaming's coverage of their 2006 Halo 2 season.
Menagerie gets special mention for reappearing in Halo: Reach, playing as Noble Six helps escort a certain Gunnery Sergeant while the Covenant glass New Alexandria in the background.
Traffic Jam. Has a driving rhythm, makes an orchestra sound as awesome as an electric guitar's power chords, and has an actual electric guitar soloing on top of that. Played during Mickey's mission where you roll through the enclosed city streets of New Mombasa in a Scorpion tank.
The reprise of "Under Cover of Night" going into the final level of Halo 3 is a standout moment from that game. To know that the two biggest badasses of the humans and Elites were about to go to their potential deaths...breathtaking. "We'll head for the Portal...and then we'll all go home."
The Epilogue Suite for Halo 2. Those guitar riffs during the first minute especially.
Wage from the Delta Halo Suite. When you listen to it, whatever you are doing becomes epic by association. Putting it with the entire suite just takes it Up to Eleven for bonus points.
From Halo Legends, "Machines and Might", a short but very sweet tune, heard in the equally sweet short, "Prototype" among other episodes.
"Oh my God. It's the Prototype suit."
"But who the hell's piloting it?"
Sacred Icon Suite 2 - basically the Arbiter's theme meets Unforgotten meets Sacred Icon Suite 1, except on steroids, on fire, and dual-wielding Rocket Launchers. That's how awesome it is.
With Halo: Reach, ashes (a real Tear Jerker to rival Unforgotten or the Halo 3 remix Never Forget) and the indescribably awesome Walking Away.
"We Remember", which is an awesome combination of Combat Evolved's rock and Halo 3's choral tunes.
Finish the Fight, the music for the Halo 3 Announcement Trailer (which was remixed into This Is The Hour, above), almost tells its own story. The Master Chief's approach is heralded by a chorus, Cortana's appearances by piano, until they meet in the same plane, emphasized by the piano chords of the Halo 3 leitmotif - which is then echoed via Orchestral Bombing as the Chief looks up to see a Covenant armada closing in on the New Mombassa portal, as an orchestral rendition of Rock Anthem for Saving the World pounds at the viewer ... and then, as the portal opens, it shifts into a variation of The Last Spartan, the Halo 2 theme, building to a crescendo as the portal lights up. Then, when Cortana whispers "This is the way the world ends," the orchestra answers, emphatically, with the Halo 3 leitmotif, which doesn't evoke annihilation so much as heroism - as if the answer to Cortana is: "No. This is the way the world will be saved." The last reprise of the leitmotif is powerful enough to bring tears to the eyes.
Technically, it's not originally from Halo, but it appears in each game: Siege of Madrigal, one of the most forlorn, haunting, and awesome Lonely Piano Pieces ever. It's pretty much Bungie's official theme. So much so that it continuously loops in the hidden tribute room easter egg on the mission "The Package" in Halo: Reach, which contains farewell messages to the community. *SNIFF*
A motion comic of the story The Return, from Halo Evolutions, was recently made. The first bit of music from it, a One-Woman Wail piece, is absolutely amazing.
From ODST, during the final level, you have Mercy Plea. So somber yet heroic; evoking the feeling of soldiers going off to face their final battle with honor.
From Reach, we have a track that plays during the beginning of Tip of the Spear, and caps off the game credits: Unreconciled. So chillingly epic.
With Halo 4, Neil Davidge proves himself a worthy successor to O'Donnell (who is contracted to Bungie, not 343 Industries), with "Arrival", an epic piece that plays during the final battle of the Campaign.
On the note of Halo 4, we have the very first title in the soundtrack, "Awakening", an ungodly mix of electric guitars, choir, and violins.
Also, the Infinity's/UNSC's Leitmotif in Halo 4, "To Galaxy". A slow build-up leading to a sweeping, heroic, but serious-sounding piece. The build-up section is used as background music for the multiplayer menu, but other segments are used in Spartan Ops and of course, in the UNSC's "We are the giants now" Badass Boast intro.
Another awesome Halo 4 track is "Faithless", an epic track with a pounding beat that plays while carving through Covenant on Ivanoff Station, and in a few Covenant-themed Spartan Ops missions.
Revival starts off with an ethereal chant that ends with a surprisingly epic bass drop, and plays when you first meet the Didact to great effect.
Green and Blue. John and Cortana. Earth and sky. Life and sadness. Whatever you read into the title, this song is a powerful, compelling dirge, and it was cut criminally short in the actual game.
Arrival, Revival, and Nemesis all received top-notch dubstep remixes for Halo 4's weaponry trailers. "Nemesis" is fast paced, with some epic drops. "Arrival" focuses more on the original's violin work with a new beat, and "Revival" is a bit calmer, focusing more on the original's singing.
Another two epic pieces are Push Through, which plays when the Chief's driving a tank to clear the Covies attacking the Infinity; and Convoy, the thumping track which plays when the Chief boards the Mammoth.
Armour, an eclectic electronic mix for the concept art trailer.
Co-composer Kazuma Jinnouchi created some of the best action themes in the game. Jinnouchi's compositions are much more familiar to Halo fans than Davidge's atmospheric themes: epic scale, bombastically orchestral and clearly influenced by Marty's works.
Atonement, the main menu theme. One section stands out is a powerful build of strings, the climax of which ends with a quiet reprise of Keep What You Steal (more or less Cortana's theme) from Halo 3.