Primarily USGIs (30s & 20s). I have 20 or so of the English mags, but haven't even tried out more than one or two (those functioned fine). I have a few of the "good" 40-rounders, but I haven't tried those at all.
I have a 120-round drum, but that's just a "toy" IMHO.
I used to like Thermolds and generally had good luck with them (except the few ARs into which they won't fit without modification), but this site has turned me into such a USGI devotee that I hardly touch them anymore.
I also used to use Orlites some, but found them to be hit-and-miss about which rifles they were completely reliable with. I still use them, but exclusively with my AR pistol. They work fine with the pistol and I use different ammo in my pistol than in my rifles/carbines, so using the Orlites for that particular application makes it easy to grab the correct loaded mags/ammo for the pistol at a glance.