Which Halo game's gameplay most accurately portrays canon combat (for your taste), I say! As we know the way the game plays is different in each Halo game, pretty much.
In this format more-so:
I'd say Halo 2's Heroic Campaign is a good representation. Tone the Elite's shields down, bump Chief's shields up a bit, increase Marine intelligence (a lot), lessen how many hits they can take, allow every weapon headshot capability and we're good!
What do you think?
[Edited on 09.18.2011 7:49 PM PDT]