Halo 2, hands down. The mains reasons being the Jackal Snipers (by far THE most OP thing in any Halo game) and the bullet sponge Brutes and Flood. There's a reason Monopolised is one of the least-earned achievements.
The other games have their moments.
Halo 1 has monotony of The Library and the rocket flood infestation on of Two Betrayals.
Halo 3 has a few sections of The Storm, the AA Wraiths on The Ark and the level Cortana, the ugliest and most confusing level in Halo history.
Halo Reach, while somewhat easy overall, did have the holdout in front of the lab in The Package, and the final section of Exodus where you fight into the villa.
ODST was, again, somewhat easy, but Coastal Highway is tough without a second player in the Warthog (I love you Buck, but you can stop shooting the dead grunts).
Halo 4 has several sections that are tough, most of them involving Prometheans, especially on Shutdown and Midnight (that flying section), but overall the campaign is more annoying than hard.