who makes the best synthetik mags on the market for 9mm ar15s…
I prefer GLOCK mags in my 9mm build. 
That glock mag is pretty slick, who makes it or is it custom?
Olympic makes the Glock magazine ARs. The only synthetic 9mm mags I know of for the AR are made by ProMag and unless they changed them they were terrible.
And in fairness to ProMag their synthetic magazines for the Mini 14 worked awesome for me. Until Ruger released their mags to the public the only after market mags that worked reliably for me were ProMag.
I see they are getting better reviews at Midway http://www.midwayusa.com/viewproduct/?productnumber=838489 but my experience with them was less than stellar. Stick with the original Colt/Metalform mags.
I have 4 of the Promags for my RRA 9mm and they seem to work fine. When I bought them the metal mag options were prohibitively expensive so I tried the Promags. I don’t shoot it much and its just a range toy for me but the wife put about 150 rds through it two days ago without any problems. That said, they do feel flimsy and I wouldn’t use them for serious use.
The wife just ran another 200+ rounds through the 4 Promags I have in a competition two days ago, ran 100% as far as feeding goes. They do not activate the bolt hold open, ever. If you are looking for cheap range mags, they should be fine.
This seems to be the way of it. Either they run in your build or they do not. There seems to be no middle ground with these.
Stick with the original Colt/Metalform mags.
+1
I’d also recommend modified UZI magazines.
Several years ago a friend of mine tried Promags and they would not feed reliably if he loaded more than a dozen rounds into them.
I’m using a Hahn block in my carbine and his mags wouldn’t work in my gun very well either.