Short, sweet (as hell), and easy to understand.

Must-Have Remakes -These maps better be out by this time next year, or somebody's gonna get tagged with a blue spider.
Halo: CE Maps
~Sidewinder*
~Prisoner

Halo 2 Maps
~Lockout
~Sanctuary
~Warlock/Wizard


Nice-to-Have Remakes -These maps would be great additions to Halo 3, but they're not an absolute necessity.
Halo: CE Maps
~Hang 'em High**

Halo 2 Maps
~Terminal
~Beaver Creek/Battle Creek
~Coagulation/Blood Gulch
~Relic


Need-a-Spiritual-Successor Maps -While these maps themselves may not warrant a remake, they have great concepts, ideas, settings that need to be in Halo 3.
Halo: CE Maps
~Chill Out
~Boarding Action

Halo 2 Maps
~Headlong
~Turf
~Midship
~Waterworks

And there's my list.

*God this map deserves online play. It's always been better than bg/coag.
** I'm not one of those snobby purists that found Tombstone to be bad because it changed too much of Hang 'em High, I just thought Tombstone played terrible, and looked terrible too. Almost like a Halo 1.5 quality looking map.