Here’s my take, and yes I am totally biased considering the M16/M4 was what I trained on and deployed with for over 20 years.
The AR hands down as far as I’m concerned. Ergonomically speaking it is one of the best IMO. Yes the CH is a pain initially but after training it is just natural to me. Add to that the offerings from Geissele, BCM, Radian, etc… with their ambi CHs and its so easy a “cave man can do it”. Modularity, again the AR. Some have caught up (newer FALs, AKs, CETME types, etc) but none approach the AR in my opinion. Ease of support and maintenance/repair, again the AR. Just look at swapping out barrels for example. Add to that parts availability. If you have a “mil-spec” AR then parts are truly plentiful and easy to get.
Now I have love for AKs, FALs, bullpups, and the such and have shot them; however, practically speaking I believe the AR is still King in this country for the reasons I posted above. Also a huge driving factor from what I see is the large number of GWOT vets who gravitate to the platform they fought with. Just as the M1 had its adherents after WWII and to an extent the M1A/M14 after Vietnam (though I would argue the AR picked up steam in theciviwian world because of Vietnam), the AR today is the face of the American shooter thanks to our time over in the sand box.
Just my to cents and I’m sure I will incur the wrath of the AKphiles here, LOL.