I don't own an AR, and I prefer 20rnd magazines for most tasks, but I own a rifle that takes AR type magazines and have tried just about every available magazine, my conclusions are as follows:
Lancer L-5s: Excellent magazines that are very rugged, reliable, and allow you to monitor the capacity because they are smoked translucent polymer. The steel feed lips allow you to keep the magazines loaded and prevent damage, the rubber base plate also prevents damage when dropped. Bar none my favorite magazine, I keep these around for serious use (defense, et al.). Available in 20 and 30rnd variants, they are priced at about $15.00USD/ea.
Magpul PMags: Another great magazine. Very durable, but feed lips can crack if dropped on a hard surface, though the have a great warranty against damage. A version is available with a window for monitoring the capacity. They have become the new standard, I use these for general range use. Also available in 20rnd version, they cost about $12.00USD/ea.
USGI Aluminum Magazines: The standard AR magazine, they are durable, but the feed lips can deform if dropped (thus causing malfunctions). The feed lips are also subject to fatigue, but have a moderately long service life. Likely the lightest and certainly the most available magazine available for the AR-15 platform. From my experience the best USGI magazines were made by Labelle, Sterling, Adventure Line, and Okay Ind. 20rnd magazines are available (NHMTG make decent 20s), but the 30s usually exhibit better quality (as do 40s but they are rare, bulky, and costly). I use these for most of my 30rnd magazines. They can be found for $8-10.00USD though some more rare examples (collectors items) can cost several hundred.
Brownell's Brand Magazines: Available in both 20 and 30 round capacities, these appear to be the same as USGI aluminum magazines and exhibit the same flawless function IME. I prefer genuine USGI if only because it is made for and/or used by our armed forces, but I wouldn't turn down one of these if I needed one. They average about $8-12.00USD.
Israeli Orlite Magazines: Another polymer magazine, these are fairly durable, but not quite as good as PMags and nowhere near as good as L5s IMO. I sold all of these after the switch to PMags, but they aren't bad. To my knowledge 30rnd magazines are the only version available. Prices range from $5-10.00USD depending upon condition (most are surplus).
Thermold Magazines: Similar to Orlite magazines, but lacking the steel reinforcement and having a lower melting point (best not to be used in FA applications), they are a bit weaker than the Orlite but afford similar performance (though some folks report problems with them). 20 and 30rnd variants are available; they are most often priced at $4-8.00USD.
C-Products Magazines: They produce both Al. and SS magazines in both 20 and 30rnd versions. They seem to be hit or miss, I have had some issues with feeding using some of these magazines and would avoid them altogether. Typically cost $8-15.00USD.
Pro-Mags: They manufacture Al. and polymer magazines in 20 and 30rnd capacity. The Al. ones can be decent, but are hit or miss. I have never seen one of their polymer magazines that would feed properly. I would avoid both, and forget the polymer ones exist. They vary in cost from about $5-10.00USD.
"USA-Brand" Magazines: Al. magazines made to the absolute lowest standards, avoid all of these at all cost. The culprit of a great number of AR malfunctions. Found at most gunshows for $5-10.00USD.
There are a few that I haven't tried, or haven't much experience with, like the HK mags, Cammenga, Korean steel magazines, or Fusil, but honestly I see little benefit in trying more because the top 4 work so well (especially the Lancer mags) and many of the "specialty" magazines are very costly.
