OK, time for some snowy Halo!
Assault on the Control Room - an iconic level (well, aren't they all). This level strikes a fairly decent balance between indoor sections (including a couple of optional stealth bits) and outside, often vehicle-based combat. Aesthetically the level is a real change of pace compared to the rest of the game so far, and the fact that the previous level was a temperate island (The Silent Cartographer) makes this desolate snowy wasteland - one quick Pelican ride away through Halo's access shafts - a real reminder of the crazy alien fusion of landscape and architecture. It's also the first time you get to pilot a Banshee, which at the time was an epic experience.
The biggest multiplayer map to ship with the standard console version of Halo:CE, Sidewinder was probably not appreciated by many players as the first Halo had no online functionality, meaning a LAN party with 15 other Xbox owners would be the only way to play this huge horseshoe-shaped multiplayer arena as it was intended. Probably best played on Capture the Flag mode with big teams, outdoor battles would be dominated by Scorpion tanks, snipers and flag-carrying Warthogs skidding around on the ice, while the cramped interior corridors and the appearance of shotgun and invisibility powerups made for lethal attack/defence scenarios. The Halo 3 DLC map, avalanche, was a remake/homage to Sidewinder.
Halo 2 has a lot of... weird levels. They really cranked up the "alien factor" on this game, and as a result there are some levels that are just quite visually confusing. Quarantine Zone is one such example, featuring ruined, radioactive-looking megastructures, Flood infestation, snow (or is it nuclear fallout?), and a cool gondola ride at the end.
Snowbound: Halo 3's first Multiplayer snow map. What I mostly like about this level is the intricate level design and the way Bungie combined gameplay with storytelling. Covenant research bases and excavations link the different areas of this level, and the outer boundary is defined by sentry guns which will shoot at people who stray too far outside - a damn sight more imaginative than a sterile and contrived "return to the battlefield" message.