The level before Cortana in Halo 3 (either the 7th or 8th stage IIRC), truly epic... the Banshee fight, the 2 giant walkers, the stage had everything that made Halo what it is, all condensed to one level.
The stage when you finally make it back to City 17 in HL2 and shit hits the fan. Striders walking around everywhere, you commanding the squads to fight, the battle with the chopper on the roof.
All Ghillied Up in COD4, like about 80% of the singleplayer in COD4 was excellent.
I may be the only person in the world who actually liked the Library in H1. Which is ironic because I expected to hate it, since it had 2 of the things I hate the most about the Halo series - repetitive mazelike level designs and Flood. But after the initial start, where you have such shite weapons, you really get into a rhythm, using a shotgun/assault rifle combo, and things just flow so well.