Halo CE is hardest for me because of the last 3 levels. Coming off of The Library is bad enough. But when your next three levels lined up are Two Betrayals, Keyes, and the Maw. The Maw giving the most relief out of the three. When using speedrunning tactics, there aren't a lot of de spawns and a whole lot of RNG to kill you with Rocket Flood. Halo CE has the Most Tenacious enemies, and that's only on par with Halo 4 believe it or not. To me at least.
Halo 2 is most likely second, but what keeps it from getting first is the amount of shortcuts you can take. You can beat the level Uprising in 2 minutes flat which helps a lot because that level Vanilla tactics a pain. "Off the Rock, Through the Bush, Nothing but Jackal" Is one of the hardest sections to ever solo legendary... Vanilla. Once again there's a skip to De-spawn that whole area and cut the level in half. So if I was playing Headstrong/Vanilla and taking on everything Halo CE and Halo 2 would be reversed. But using Smart/Coward/Speedrunning Tricks makes Halo 2 the easier game. Depends on the way you play it.
Halo 4 is next. The Vehicles being made out of Paper Mache really doesn't help in any vehicle based part, minus the Mantis area on Infinity. Midnight, The Marine Achievement Hill on Infinity, Forerunner, and Not Skipping Shutdown can make this a pretty difficult game. Not to mention the Bullet Sponge Prometheans, Who I still personally like just for the Purpose of Punching right in the mouth and their Crispy fried deaths. Moderately tough game. Not as easy as people think. Especially By Yourself.
Halo 3 is what I said was easiest. Granted there are some pretty difficult parts near the end of the game. I have played the game enough though to get past these parts pretty fast. There is also a reasonable number of shortcuts like Killing the Scarab on the Ark the Moment it drops off the Structure. Killing the Scarab on The Storm the Moment it drops with a gravity lift. Ecetera Ecetera. These Coupled with the Crumbly Nature of the Flood in this game, comparatively speaking to it's predecessors, makes Combat Forms a Joke. The Tank forms are a damned Joke Honestly. The Ranged Forms are the Hardest thing they have to offer in this game. I.E The Annoying Cortana Mission. I Still miss them so much though
When I went for beating the Original Trilogy in Under 3 hours for each game, Halo 3 gave me the least fuss. So Halo 3 is the easiest to me, yet still my favorite campaign, Gameplay wise.
Halo 3: ODST I don't have much experience within the campaign, but I'd say it would probably Tie with Halo 4 if I had to venture a Guess. Not too hard, but not too easy. I'll have better judgment on this when I play it on Legendary for myself.
Halo 3: ODST
Also, If Reach was on the list, I'd say that one was the one that gave me the most fuss of all the Halo games. The Pillar of Autumn is such an unforgiving level. It may have gotten easier because the last time I played that game Legendary by Myself was in 2010. Oddly enough though, it's the only Halo so far I have completed on LASO.