Well I don't have one from each game as I'm trying to remember each game's campaign lol but here's my list
Halo CE: The Library - I think this is probably on everyone's list. The Flood just keep coming and it's annoying when you have to wait for Guilty Spark to unlock the doors as you don't really have a lot of space to manoeuvre
The Two Betrayals - I last bit is more of a nuisance since you are on foot against vehicles especially at the end on the open bit where the Flood and Covenant are and you need to steal (or rather die trying...a lot :) ) a banshee between two wraiths and hunters whilst flooding are coming in from all sides.
Halo 2 - Everything on legendary lol. In all seriousness though:
Regret - The Gondola rides and the jackal snipers make this a pain. The actual fight with the prophet seems (to me anyway) to be very luck or RNG based.
Gravemind - You spawn with a needler... imo, at least on lower difficulties, it's not a bad weapon against brutes...until they start charging (if you don't have enough distance you may not have time to get the supercombine). On higher difficulties, in particular, you really need to be on the ball here otherwise you just get charged at and die. In general, I find everything about this level somewhat tedious from the Brute Shots to the rocket flood to the final part in the mausoleum. That and it's one of the longest levels ever.
Halo 3: Cortana (probably everyone else's) - the flood swarm you in this level, the tank forms are tedious even on lower difficulties unless you have an energy sword.
Halo 3: ODST - Never played it :(
Halo Wars: Arcadia City - a relatively large difficulty spike imo compared to the first three levels. Annoying due to the fact it seems very RNG and that the Covenant are already fully upgraded and in the last few minutes seem to come at with you everything.
Halo: Reach - Can't really remember too much of Reach's campaign tbh but going out on a whim Nightfall - it's supposed to be stealthy with the sniper and all that but often I find myself messing up somewhere and just going gun's blazing.
Halo 4 - Midnight because of all the knight's and their annoying teleportation wizadry and all their other insane abilities and the flying sequence in the first part.
Shutdown - namely because of the Gondola ride
Halo 5 - The Breaking - Promethean weapons track you in this game and there are a lot of them. The fight with three wardens is extremely annoying and tedious (especially after you have fought him multiple times before in the campaign) though I suppose it thankfully is skippable.