My vote goes to chillout.
I can’t really give an opinion on all the maps, as I started playing online multiplayer in Halo 3. However, the first map that springs to my mind as one that was always enjoyable is Guardian.
Forgeworld, Sandtrap, Valhalla, Standoff.
Just 1 pick, what would it be.
Guardian
bloodgulch
LOL at bloodgulch
Danger Canyon 
> LOL at bloodgulch
I lol’d at it too because the topic could be closed now, of course the best map is bloodgulch. It defines halo’s multiplayer and is the go to map.
guardian
> Danger Canyon 
Best. Map. Ever.
+1
Best map overall - Bloodgulch
Best competitive map - Lockout
Midship or Guardian
The Pit from Halo 3
Avalanche! 
Ice Fields.
Guardian
Turf.
Can’t go with just one, but i’ll list my favorites from the entire series…
Warlock
Hang Em High
Midship
Lockout
Headlong
Waterworks
Guardian
Sandtrap
Ghost Town
Last Resort
Zealot
Ivory Tower
Powerhouse
Burial Mounds
District
Avalanche
Rats Nest
Omg… can’t decide just… one… OGFJFJFIHJFHF
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