Hahaha, I didn't expect my comment about Halo 3 maps to start a huge discussion about map quality.
Pointless to talk about Halo CE maps since all the best ones have been remade and the rest wont even work with a properly balanced weapon set. (ie. no pistol)
Halo 2 had some great standout maps, and like someone already mentioned, the majority of the memorable ones were asymmetrical maps designed for objective games. Zanzibar is the obvious one, but Headlong and lots of the DLC maps like Relic and Terminal were utterly fantastic too.
The problem I have with a lot of the Halo 2 maps is that some of them werent quite big enough for proper objective based games, but they werent symmetrical enough for really fun, competitive, balanced slayer games. Lockout gets alot of love, but it is such a freaking cinch to just dominate that map once one team has a couple BRs and a sniper rifle. Ascension was downright broken for most of its existence. Midship was a great slayer map, by contrast. Ivory Tower wasnt bad either, despite being really really ugly.
The best maps in Halo 3 are also asymmetrical. Obviously Last Resort, which is an enhanced remake, but High Ground is a fucking blast for objective gametypes (not so much slayer, when the spawn points can get screwed with). Like Halo 3, lots of great asymmetrical maps came in the DLC too. What separates Halo 2 and Halo 3 for me, is that there really arent many maps on Halo 3 I absolutely cant stand, and I think thats because Bungie made the large, symmetrical maps a lot better this time. Granted, some of them are remakes, like Valhalla and Avalanche, but they are greatly IMPROVED remakes.
When I said Valhalla had more 'routes', I meant there are more options for crossing the map safely, without getting gunned down by snipers parked at their side of the map. Cover provided by differences in elevation through the middle and near the river, as well as the rocks on one side and the caves on the other.
My favorite maps:
Valhalla
Last Resort/Zanzibar
Headlong
Avalanche
High Ground
Containment
Midship
Rat's Nest
Standoff
Ghost Town
The Pit
Standoff
Ivory Tower
Relic
Terminal
Again, keep in mind that for me objective modes > slayer
Longshore