Magpul PMags and TangoDown ARC mags have shown themselves to be the best on the market for not only durability, but also reliability. Reliability is key. I have had significant success with both.
Most metallic mags are fine for a short time, but they tend to have a shorter service life. The updated USGI mags with the green "anti-tilt" follower are key, but even those followers are pretty marginal. They still tilt. Such is why the Magpul follower upgrade is a good idea for USGI mags.
HK mags are by far the best metallic mags on the market. However, I think HK forgot that mags are a wear item and are meant to be cycled and replaced at the first sign of malfunction. HK decided to charge $50-56 for each magazine, which prices them out of legitimate contention by all but the most dedicated HK fanboys.
Lancer L5 mags have gotten decent reviews, but keep in mind that clear polymer is not as strong as black polymer. There's some info online about the strength of different colors of polymer that can explain it better. Lancer has done a good job with the durability enhancements on the magazine, though. Kudos to them for the steel feed lips.