USGI, Thermold (US made only), Orlite, probably in that order of desirability.
Note that you many have to bevel the ridge on some Orlites to fit into Bushmaster guns.
Best place to buy mags - RIGHT HERE! Since you found the INDUSTRY forums, I assume you can find the TRADING forums. Look in the mag forums and you can typically find the best deals.
DO NOT BUY USA MAGAZINES. DO NOT BUY METAL MASTERS, OR ANY OTHER COMMERICAL MAG. NO STEEL MAGS. THEY ARE ALL CRAP.