The ZM is an interesting AR and I've come close to buying one a couple times but I really don't see the advantage of a folding stock, not for civilian use anyway and I would want a reliable parts supply available which I doubt is the case with ZM since they are not very popular and have a few proprietary parts . I've had other black rifles with folding stocks and they never get folded, my Swiss only got folded when I was showing someone that it could do it and my SL8 with Hera lower and ACR stock can fold but I doubt it will ever be folded other than when showing it to someone.
To me the best AR is a reliable one, I don't really care what brand or what gas system, I've owned Armalite's, Bushmaster's, Spikes, and now own a couple PWS. To me my favorites are my PWS with their light weight long stroke gas system but that's just me, I also like my 18 inch DI rifle, all of the AR's I've owned have been extremely reliable and have been more accurate than any other black rifle I've shot. My 12 inch PWS would easily outshoot my Swiss classic green with 55gr FMJ. Any AR can be made accurate with a new barrel and anyone with basic skills and a couple special tools can swap out any parts on an AR without the need of a gunsmith.
If AR's weren't restricted I doubt very much anyone would buy an XCR, Swiss or any of the other options. I was going to add more expensive non restricted options to the previous statement but there are some pretty expensive high end AR's that are just as much or more money than a Swiss.
That was my primary reason for buying them, I like having a non restricted semi auto in 223 but if I could carry my AR in the bush I wouldn't need these other more expensive, heavier, and less accurate rifles.
I'm leaning towards an ACR next but would want one converted to non restricted. I don't think it does anything better than an AR but I like the quick change barrel option as I would like to have a non restricted 223 barrel for varmint hunting a non restricted 300 Blackout barrel for close range deer hunting and a 12 inch 223 barrel for playing at the range.
As was said earlier the best AR is the one you own as long as it's reliable.
My favorite thing about AR's is that if you get tired of it in it's current configuration you can always change it without having to spend a lot of money. A new upper can be bought for a reasonable cost, a new barrel is only a few hundred dollars and a new stock is under $300.