What does the AR win? I will definitely say that the AR has won the most customized .223 rifle out there.
How do you want to define the terms of winning?
A lot of guys like to define it as the number of armies that have fielded a particular gun, or how many of them have been built since their inception.
I'm not in the military, nor will I likely be in a situation where I will have to run a gun that is issued to me at any point in the foreseeable future. As such, I suppose comparing logistics numbers is fun, but I don't find it particularly enlightening or practical when attempting to quantify what is best for my particular needs.
I much prefer to look at those shooters who are extremely good, but demographically similar to myself.
If we look at a cross-section of the top competitive shooters in the disciplines that allow usage of semi-auto rifles, there's a pretty clear preference for the AR. ARs are the dominant choice for Service Rifle and National Match Rifle competitors at Camp Perry, and in fact have been used to set records in a shooting sport that has been around for the better part of a century.
If we look at newer shooting sports like 3 Gun, the AR is far and away the dominant choice of not only most of the people who choose to compete, but of those who win.
I've even seen a couple of AR10-pattern guns at an open-terrain Sporting Rifle match with targets at distances of 400-875 yards.
Ok, well, what about hunting? As has been previously stated, a lot of varmint hunters report good success with AR-pattern guns.
Other than that, it seems like there are a growing number of hunters who are using AR-pattern guns chambered in heavier calibers for hunting big game, and with good results.
How about for martial and defensive use? For issue to patrol officers and SWAT cops, the AR seems to be pretty ubiquitous. I'm not a cop, so this data point isn't necessarily completely cromulent to my situation, but it's worth noting.
What about people who are concerned with personal protection and defense? Well, most of the well-known trainers like Larry Vickers, Pat Rogers, and the MagPul Dynamics guys seem to favor the AR for their classes, and there are a number of reports that the .223 round is a good choice for urban environments as it will have less of a likelihood of over penetration compared to other calibers, including some handgun rounds.
Alright, so what if you don't want to be concerned with all of that heavy competition and personal defense stuff and just hit the range for a bit of plinking? Well, toss on a .22 upper, and you've got a great gun for drilling tin cans and teaching little Timmy the basics of marksmanship.
I suppose you could argue that the 10/22 is more modifiable than the AR, but if so, it's only just barely, and ultimately that doesn't matter so much because the fact that it's limited to only rimfire options means that the applications for the 10/22 are rather limited, whereas this is not the case with the AR.