As others have said, Jackal snipers in Halo 2 make levels that would otherwise be fairly easy extremely difficult.
If it wasn't for those I would have said Halo CE. You die in about 3 plasma rifle shots, and it can take 2-3 clips of pistol or AR ammo to kill an elite. They also have a nasty tendency to kill you in a single melee. Flood with shotguns and rocket launchers are quite a treat as well.
But the other folks are right, there's nothing quite like killing a bunch of elites and hunters, going "Hmm, looks clear out there..." then walking out and hearing the beam rifle noise and knowing that you are about to get sent back to the last checkpoint to do it all over again.