I like most of the maps, but my favourites vary depending on the game mode.
For assault and 1-flag CTF, Zanzibar rules, and Ivory Tower is good for smaller games as well. I have a love/hate relationship with Headlong, when things are going well i love it, but when things are going badly, i hate it :lol (i probably just need more practice on it).
For multi-flag CTF, Coag is good, but i love CTF Wraiths on Waterworks, so that probably comes in as absolute favourite. I quite like Collossus, too, when people aren't bouncing the flag around like muppets.
Lockout and Ascension i only really like for (team) slayer, oddball variants, and things like KotH and territories.
Territories on Midship, with a single territory, and a ten minute time to win, is utterly insane, but a bit random for serious games. Great to mess around with, though

(had a game with those settings that my team ended up winning 10:00-9:59, after being down by about 7 minutes to 3 earlier on, talk about intense...)
Burial Mounds is good for big team slayer/1-flag ctf/assault games, and is also great for team slayer games with no shields, no radar, damage resistance on, no powerups, bonus points on,, no vehicles, no turrets BR and magnum only, no grenades. Brilliant long-range cat-and-mouse headshot action
And i second the call for Sidewinder. More big, open-ish maps plzkthx bungie.