Do you mean the Silent Cartographer or Assault on the Control Room? Assault on the Control Room is my favorite.
Probably Halo 2's The Arbiter. I don't know what it is..but something about it.... Other than that, The Silent Cartographer from CE and The Covenant from Halo 3 are also great levels.
I just love the level Halo because of the boss battle. Wrong. I really love it because of the epic Mongoose drive.
Me and my friends only play The Covenant and Halo (Rally Point Alpha) when we do campaign. I like the Covenant because its the only level of Halo 3 that closely resembles a Halo CE Mission and cranks it up a notch, and we play Halo just for the driving. We can laugh for hours at how close we get to the finish, only fall to our deaths. Epic lulz.
Halo:CE: Truth and Reconciliation, Two Betrayals( the mission before Keyes.) Halo 2: Sacred Icon,Delta Halo,Uprising Halo 3: The Storm, The Covenant, Halo
Halo 2: Outskirts/Metropolis Halo 3: Covenant/Halo ( the driving part, prolly most epic part of the game!)
My favorite is Tsavo Highway. You can get in a Wraith and have fun, explosive chaos. But that's on Normal or Heroic. On Legendary, I like it because you have to hold postions often, with challenging opponents. Teh, screw Easy.
I really like all the vehicle use in the covenant. It's a fun level and is really long. The silent cartographer is fun tho too.
My favorite Mission: The last mission Halo Why?: Because of the last part where you have to get in the vehicles its so much fun especially on legendary.
I like The Ark or The Covenenant because I like driving vehicles, but Halo is always good when your in a 4 person party on the ghost, chopper, mongoose, and warthog yelling into the mic for your life.