"Panther" from 4, which is perhaps one of the best songs for an opening stage, and comes back again in "Marche Au Supplice", which pits you against four of the best pilots in the game, and even without any allies on Hard, and the song only escalates the battle into a Moment of Awesome as you hear the enemies' disbelief if you begin to take them out.
Possibly the best moment of this mission is actually in the normal mode; if your partner gets taken out your client for the mission comes over the radio with a retreat order, and immediately after that your operator, who is normally cool and detached, comes back with "NO! Continue the mission... I know you can do this."
"Ambiguity" from Armored Core 1 is downright beautiful. It perfectly captures the tragic nature of every Armored Core world, and the sometimes terrible things that the player does to get ahead in those worlds.
The song "Day After Day", which plays in the "No future for mercs" trailer. While this song doesn't have a studio version out, it does have a live version, and there is a cover which is almost as good as the real thing.