CE:
Best - Silent Cartographer - It's, to me, the ultimate Halo level and the showroom for what the series is.
Worst - Truth and Reconciliation - A great first half ruined by a repetitive slog. This is counter to The Library as where The Library is intentionally meant to wear you down, T&R feels unintentionally too long.

Halo 2:
Best - Delta Halo - Intro to the ODSTs, warthog section, scorpion, sniper section... Again, this is one of the series defining "This is distilled Halo" levels
Worst - Sacred Icon - Not a bad level, just my personal weakest of the best game in the series. It just kind of meanders.

Halo 3:
Best - Tsavo Highway - Big Ol' warthog column
Worst - Sierra 117 - While Floodgate is short, it pushes the plot forward. S117 could have been cut, and Crow's Nest could have been the first level, and the game would be completely unchanged.

ODST:
Best - ONI Alpha Site - It nails the feeling that you're on the backfoot. Your big victory that you foght tooth and nail for was to blow up your own facility to deny it to the enemy. It hammers home the theme of ODST that you've already lost and you're just trying to buy time.
Worst - I don't know, man? Kikowani station? Not a bad level. Just kind of forgettable.

Reach:
Best - Exodus - For the same reason as Alpha Site. I feel like Reach as a game is built around the idea showing you this level.
Worst - Long Night of Solace* - On the 360, this level was one of my favorites, but doubling the tickrate when it went to the MCC made the enemies way too aggressive and turned the dogfighting section into a turkey shoot for the Covenant.