Never leave home without your shotgun.
I agree that Halo 3 didn't really have many really hard points. I'd probably pick.. I think it was The Ark, or whatever the level before Floodgate was. With the AA gun. Gods that thing's guards were a pain.
Halo CE was a close one between The Maw and The Library. So many Flood..
Halo 2 was.. hell, the whole damn game on Legendary was evil. That first level where you're Thel, though, with the Flood. Evil, evil level.
Reach was, without a question, The Pillar of Autumn. So many high-rank Elites out for your blood, then the MAC gun sequence.
ODST was pretty tough the whole way, but it was fun so I didn't notice much. Data Hive was likely the toughest. I hate drones.