I own a Bushmaster AR Shorty. The following is my pick list of mags (in order of reliability). Mag that did not make the list, I consider crap.
1. USGI 20 rounders
2. USGI 30 rounders (All but Coopers) Coopers are poopers.
3. *Israeli Orlite (I find myself using these most) see note.
4. *Thermold
*Note: Are as reliable as USGI as long as you do not keep them fully loaded for long periods of time. They seem to swell slightly.
Lower cap mags
1. Colt 9 round
2. Armalite 10 round