There have been soo many awsome levels on campaign for so many different reasons so why not share yours.
For me nothing can beat halo on halo ce, just getting the game after its been released back in the day and getting blown away by how big the level was, being 11 years old it was a huge thing!!!
But covenant on halo 3 was awsome too 
assault on the control room from halo:ce
this is a tough one will get back to you on this so many campaigns
Welcome and happy hunting Spartan 
Hey Micky, welcome to Waypoint!
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Welcome to Waypoint 
Happy Hunting Spartan! x
Exodus from Reach was the best. I donβt know why but crumbling buildings in an urban warzone always make me happy in a game like Reach.
Also when you climb into the falcon and get to shoot all the brutes and banshees as you fly to the beach to activate the anti air missile pods and damage the corvette, giving the civilians enough time to escape New Alexandria was a great part too.
For me I think the best 3 levels were all in halo 3. They were the covanent, Sierra 117 and the ark.
I think the soundtrack in a level has an effect on my opinion.
but then again, I think H1-2-3 had the best campaigns with the best missions overall
I loved destroying Scarabs in Halo 3. It felt so epic
I couldnβt pick just one but some of the missions that are brutally hard on Legendary (Truth and Reconciliation, Cairo Station, Gravemind, Long Night of Solace, etc.) are actually a lot of fun if you dial the difficulty down to Heroic or even Normal. Theyβre generally linear and have lots and lots of stuff to shoot, which is ultimately what I signed up for.