Halo 2/YMMV

Awesome Music: Halo Theme Mjolnir Mix: take the original theme and make it even more awesome but adding ELECTRIC GUITARZ!
Best Level Ever: Metropolis. Drive a tank across a bridge fighting Ghosts, Wraiths, Banshees and Phantoms, followed by a battle in the city after a quick battle in a tunnel, and the level ends with the Scarab fight.
Even Better Sequel: It is debated on how well the story lives up to expectations, but this game is considered one of the best shooters of all time.
Game Breaker: Though not supposed to be something that players have access to, the Scarab Gun makes the level it can be found on completely trivial.
The Scrappy: Miranda Keyes to some extant, going in head first gets quite annoying at times. Especially if you are, well, surrounded by flood parasites. The EU explains it a bit. Apparently she's trying to live up to her father's name.
Scrappy Mechanic: The rank system was surely the least secure aspect of ranked playlists, deranking was a notorious glitch in the system or result of a really skilled hacker. God bless the heart if anything more than twelve rank levels have disappeared off your record.