Most of the M16A4's I've seen have ACOG's. M16A2's get Aimpoints (M68/CompM4) on a cantilever mount out over the handguard. I've seen M4's with both, and some EOTechs, on them. The EOTechs were personal, but no one said anything about them. Additionally we usually have at least one AN/PVS-14 for every one on duty per shift, that either goes on your helmet or on the gun behind the M68. M249's in the Automatic Rifleman role tend to get ACOGS. M240's get ACOGs or an Elcan M145 Machine Gun Optic. I've run M2's with either Trijicon's M2 specific ACOG MGO or AN/PAS-13 Thermal Weapons sight. Those are the optics an Army Reserve Trans unit has on hand and on weapons. And we are normally pretty far behind the cutting edge of new gear.
I've even known guys that had both an ACOG and an M68 zeroed for their weapon and would swap them based on the mission. They were combat arms along for a ride though. We transporters generally have to pick one and stick with it.
ETA: I'e seen Navy and USMC M2's with EOtechs on them as well. Don't know how well they work, but they had some kind of fancy recoil reducing mount that housed the HWS and a scope of some kind for target ID.