Mec-Gar is the only non-factory mag you can count on, and they are the OEM for a lot of factory mags.
ProMags I wouldn't buy unless from a place like Midway that will take them back when they don't work. I've had more ProMags that didn't work that did, but the good ones have worked very well, the bad were bad from the git go.
Tripple-K only as a last resort -- they make lots of mags for old guns, that are otherwise very hard to find, you might get one that works, but they are also usually priced as high as Mec-Gars and never as good.
National, only if priced per lb as scrap.
--wally.