I approve of the epic longpost tangent we're going down. *munches some raisin toast and cracks fingers*
Halo (H1) - The first step onto the ringworld was very cool. Multipath design and large scale combined to make this level very memorable.
Truth and Reconciliation (H1) - The opening sections were the best part of the level, with great visuals and music. The sections onboard the ship were fun too, if not a bit repetitive.
Metropolis (H2) - I always liked the bridge encounter in this level, and the vehicle combat is good too. Taking down the Scarab is fun as well, if not outdated now
The Arbiter (H2) - [cloak][stab] Rinse... repeat. Good stuff!
Delta Halo (H2) - The architecture was always very cool, in a space-Aztec kind of sense, and I like the encounters and vehicle combat.
Quarantine Zone (H2) - One of the better flood levels we've seen, thanks in large part to the vehicle and open terrain elements.
The Great Journey (H2) - I like the large scale and Banshee combat, but I gotta say the boss battle being the final act is kind of... off.
Silent Cartographer (H1) - It was one of the best levels of the game hands-down. Great opening (the beach battle insertion), very little repetition, and a good multipath design that nudges the player forward and still provides a large and diverse battlefield.
Assault on the Control Room (H1) - Very large level with some very nice vehicle segments. Repeating rooms and bridges brings it down somewhat.
343 Guilty Spark (H1) - It's the best (and also the first) Flood level in any of the games. The Library took the concept and ran it into the ground. Halo 2 and 3 did much better (Cortana is so much better designed in every single way than that starturd that is The Library). A slow reveal of the Flood is something I missed in the following games. Yea, we know they exist and all that, but I would have preferred to see a more suspenseful introduction in Halo 2 and 3.
Sierra 117 (H3) - Very beautiful level with a lot of fun battles. The dam segment is easily my favourite part of the level, the multiple approaches and the battles with the Chieftain and the reinforcements are always good.
Tsavo Highway (H3) - Warthog and Chopper driving sections are always fun, and this level has a series of good encounters and setpieces.
The Storm (H3) - Debut of the new Scarab, and a nice mesh of indoor and outdoor combat. If The Library's interiors were designed nearly as well as The Storm's interiors, I wouldn't hate it nearly as much.
Floodgate (H3) - Probably one of the strongest Flood levels in the series. The level is gorgeous, with a lot of interesting arenas and the debut of the ever-lovable Flood Pure Form.
The Ark (H3) - I love the scale involved in this mission. Large vehicle battles mesh with infantry combat very nicely, and the player gets to use so many different vehicles over the course of the mission.
The Covenant (H3) - I love this mission for the same reasons as the last, huge scale and a lot of awesome vehicle battles. 2Scarabs1Spartan is also a kickass encounter.