Halo CE: Timberland (PC), Death Island (PC), Gephyrophobia (PC), Ice Fields (PC). Yeah, that's 4 but i couldn't choose just three. Oh and i gotta go back working on that Gephyrophobia spiritual successor idea i had...
Halo 2: Zanzibar, Ascension, Turf. It is unfair to choose just three Halo 2 maps, its maps were so good.
Halo 3: High Ground, Valhalla... maybe Cold Storage or Heretic? It is very hard to say the third one, i really love Epitaph and Guardian and Blackout and Standoff for how they look if not for their gameplay.
Halo Reach: Powerhouse, Tempest... No third favorite. I'll add Spire and Highlands for i love their settings and how they look even though they ain't great gameplay-wise.
Anniversary: Breakneck/Headlong, Beaver Creek/MLG Battle Canyon, Timberland/Ridgeline. Anniversary as it's own entity for its maps are something above and beyond Reach's.
Not saying Forge World as it isn't proper map in itself and there are no truly awesome original Forge maps.
And congrats Gabo.