Magpul MAG571-BLK PMAG GEN M2 MOE Black Detachable 30rd 223 Rem, 5.56x45mm NATO for AR-15, M16, M4

Magpul appeared out of nowhere a few years back and quickly became the industry standard bearer in AR-15 mags and related accessories.  Now their products and especially their AR/M16 mags are considered among the best in the industry.  Take a look at these Specs -- Features •Impact and crush resistant polymer construction •Constant-curve internal geometry for smooth feeding •Anti-tilt, self-lubricating follower for increased reliability •USGI-spec stainless steel spring for corrosion resistance and field compatibility •Textured gripping surface and flared floor plate for positive magazine handling and easy dis-assembly.

You can buy more expensive mags, but you can't buy them any better.