I heartily suggest the Bulgarian polymer 30 and 40 round mags, as well as the new production slab sided Tapco models.
The bulgarians are relatively cheap and have metal reinforcements in the feed lips, and Tapco recently added more glass fiber to their glass reinforced nylon recipe, as well as tweaking the recipe so as to provide more hardness (Ie, less scratching)
The older Tapcos are good too, but the new ones have been fantastic. The burnt orange Bakelite looking ones are incredibly sturdy.