Personally, I beleive our current weapon setup is what I would choose.
5.56 for infantry
7.62 NATO for designated Marksman and machinee guns.
But it depends on what doctrine of fire you subscribe to.
I have read studies that say that whoever gets the most rounds downrange in the shortest amount of time is usually the one to win a military engagement. This makes sense on a few levels, more rounds incoming on the enemy reduces their effectiveness to return fire. the 5.56 with its low recoil and high capacity allows soldiers to put many more rounds downrange effectively than the average soldier could with a heavier round. The 7.62 has the ability to extend the engagement range and also turn much cover into concealment.
I know there are more "effective" rounds than what we are currently using, however "effective" does not always necessarily come down to wound potential for an on target hit. Until we come up with a military round that has the range, wounding potential and penetration of the 7.62, but the recoil and high capacity of the 5.56, I think we are on the right track.
there are certainly those that would disagree, but thats because they prescribe to the accurate, low volume, method of engagement. This doesnt make that opinion wrong in any way, I just dont beleive there will ever be one perfect round for military use, and until there is, its all going to depend on your opinion of correct engagement.