This is purely a thought experiment, since I don't really play Reach or, by association, Firefight. But I'll be damned if Halo 1 didn't have some great spots for Firefight. You can only choose one for poll purposes, but feel free to list others for the wish list. Also, I don't really have a problem with a Flood-enhanced Firefight map when we're talking about the Halo 1 Flood. Shoot off the limbs and have a parade.
Halo: CE was very, very Firefight friendly. I'd use "chock full," but that simply does not do it justice. :-)
Halo - the Chief's crash site, any of the four downed dropship locations
T & R - under the belly of the beast and into the ship's interior (maybe a way to move back and forth between them?), the hangar bay
Silent Cartographer - the top of the island (the hatch that opens for the Pelican) and the surrounding grassy areas
AotCR - any of the canyons with bridges overhead, but especially the final canyon and ziggurat
343 GS - the hectic last section near the tower (retconned to include Covenant in the melee, as well)
Two Betrayals - zomg last battle
It's a really close race between Halo, T & R, and Two Betrayals for me, but I have to go Halo because Certain Affinity is only delivering one, so it might as well be in the most iconic space it can be.