I want to rebuild some of my older USGI 20 and 30rd mags and have seen 2 or 3 types of springs... Stainless, CS, and maybe on other one, I'm not sure.  But which is going to be the best in terms of relaibility and last the longest?  Thanks!

CS is less likely to take a set, which means it should be more reliable, but can rust and needs periodic maintenance.  Stainless is more likely to take a set, but less likely to rust.  Both CS and stainless are less likely to take a set than piano wire, which the standard magazine spring.  I personally think stainless is the best compromise, mostly because of the lack of maintenance and slight performance gain over standard mag springs.

There is a brownells tech article going over the three different types of mag spring material, and their strengths.  They also sell a 10 pack at a nice price.  If you rebuild the mags, get the magpul follower as well.  dsgarms.com sells a enhanced mag which is basically a labelle mag body with a stainless spring and a magpul follower.  That's pretty much the pinnacle of performance and reliability for alluminum bodied magazines.