The 1st Cav is the obvious choice of course, it's one of the few units of the War to have fought in all four Corps Tactical Zones and in ALL the major and secondary theater of Ops I mentioned above (it even fought in the Mekong Delta in 1968-69, forming a NavCav unit to operate in the numerous waterways of the area, so we can even create a few riverine scenarios for the campaign and still be perfectly historical
)