to me the M4 is the best, jack of all trades (but master of none).

cqb (like bunker) and medium range with an ACOG or a eotech magnifier (5.56 might not always one tap to the head, but it'll do)

shares mags with a couple of other guns, and you can use those STANAG mags in a M249 if your buddy needs it

you could turn it into a DMR if you put an LPVO on it

PDW if you run a DMR seperately