My info about AR mags are mostly the 30 rounds type because they're the most common and my info are from various AR sites, but I will comment on my own experience in my summary.
Good Mags:
HK SA-80
Factory teflon-finished USGI-type mags:
La Belle
General Stamping
D&H
(Made for a number of OEMs such as Armalite, Bushmaster, DPMS, Rocky Mountain Arms, Tapco and others.)
USGI aluminum with Mil-Spec moly dry-film lubricant finished mags:
Colt/ NHMTG/ Okay Industries/ Universal Industries - (same company)
C Products LLC
Adventureline/ Center Industries/ Parsons - (same company)
D&H/ General Stamping/ La Belle - (same company)
Sanchez
FN
FN FNC mags (steel)
Sterling AR180 mags (these are combo AR15/ AR18/ AR180 mags in 20/ 30/ 40-round capacity)
British steel SA-80 mags:
Imperial Defense
Royal Ordnance
Radway Green (plastic floorplate)
FedOrd/ PMI steel aftermarket mags
Magpul PMag (new just coming out now - very tough and reliable plastic mags)
Not so good mags:
Thermolds (US made)
Thermolds (CAN made)
Orlites
Don't Waste Your Money:
Eagle
Millet
ProMag
RamLine
Triple K
USA
Victor
Western Metal
All these are aftermarket crap mags, don't waste your money.
Cooper (USGI - some are fine, but many Coopers are out-of-spec and should be avoided)
Summary:
I have a very long time 30+ years experience with Colt alum. 30 rd mags with the old black plastic USGI followers and had never have any problems with them.
In the last couple of years I also got some D&H teflon finished mags from DSG Arms (TX), C Products LLC stainless steel and HK gen 2 mags, they're all pretty good.
Steel or aluminum - if you're a heavy duty and rough on your gear user the steel mags are a better choice, for general use aluminum mags are light and works fine.
20 round mags are better for target or bench shooting because it doesn't hit the ground or top of the bench when shooting.
Followers - I would highly recommend using Magpul enhanced anti tilt followers, even the improved green USGI followers could rarely tilt and cause a jam.
Springs - USGI steel and stainelss steel springs are good but if you want to increase the reliability and life of your mags, use extra power springs like Wolff, etc and chrome silicone springs.
Other good optional accessories are the Magpul regular magpul loops or Magpul Ranger plates placed at the bottom of the mag to make removal of your mags from mag pouches and carriers easier.
There's also the Magpul L-Plate which is a rubber like bottom pad to protect your mags when dropped on hard surfaces.
Tip: Only load your USGI 30 rd mags with 28 rounds so it would be easier to load it in your rifle when your bolt is closed. Other mags like HK, C Products stainless and Magpul new PMag will take 30 rounds and would still be easy to load on a closed bolt.
Ref:
http://www.magpul.com/
http://magfaq.tripod.com/