I have to STRONGLY disagree that HK is a “good” one!

They are notorious for improper heat treating on the feed lips and weak springs that fail.

Also, Bushmaster and DPMS never manufactured magazines. They are simply rebranded magazines manufactured by other companies.

Almost any undamaged, used , military mag can be re-habilitated by first cleaning it internally, and then replacing the spring and follower with new units.  The spring should be bought from a trusted vendor/mfr, and lightly coated with suitable oil--I prefer CLP, but YMMV. The latest gen of MagPul followers are considered the best for 30-rd mags.  At minimum, a Green USGI follower on 30-rd mags.

Many people are unaware that magazines, just like their rifle, require periodic maintenance.  Mags, in AR rifles, get dirty quickly, and that dirt acts as a moisture/rust magnet with respect to the springs.  Springs rust, and rust kills springs over time.
Given good quality springs, and good quality mag bodies, mags can be stored, loaded, indefinitely without spring failure.  Springs wear out being cycled, and with wear against mag bodies.  In addition, there is always rust lurking.

Back in the day, the original grey, Teflon finish could be had by spraying the thoroughly-de-greased mag body with a spray made by PermaSlik (SP?)  There's threads about it in the archives.  Look for topics about re-finishing magazines.  Still strikes me that internally spraying the mag bodies with PS might not be a bad idea, as the resulting surface was OEM, and slippery for the springs to rub against.  Spray the outsides with whatever you want.

USGI 20-rd mags will almost always require new springs, unless already replaced, but never the replacement of the (hopefully) undamaged metal follower.  Never heard a complaint from a user of an undamaged USGI 20-rd mag whose spring was replaced with a good, new one.  There's a reason why 20-rd GI mags are valued.

Always compare the old springs with new ones bought from trusted vendors.  The usual advice is to replace older 30-rd springs whose length is less than two coils in length compared to new springs.  Not certain about 20-rd springs.  I'd suggest buying some new 20-rd springs, again from a trusted vendor, and comparing the new springs with what you have.  Certainly the 2-coil rule is cause for replacement, but other than that, I am uncertain.

Springs being a unique item, the presence on them of any sort of rust-pitting is an instant disqualifier for any sort of serious use.  Rust-pitting is a serious problem for springs as thin as those used in magazines.  Mark all your mags uniquely, and segregate suspect mags so that you can use them at the range, and repair them as needed.

It is only a matter of time before mag springs and followers--not to mention standard-capacity mags themselves-- will be restricted, so consider repairing/upgrading what you already have, as it is generally less expensive to do so than buying new mags.  Spare parts are a sort of insurance policy, so to speak.  YMMV.

In all truth, magazines are considered by the Mil as "expendable" items, both due to exigent circumstances, and their inherent fragility.  The reason why one sees so many "used" GI mags is that the Mil considers it cheaper to sell them off than to repair them.   Most of them, barring feed lip damage, or serious body damage, are easily repaired by a good cleaning, and replacement of spring, along with a modern follower.

Remember one thing:  No magazine is certified "good-to-go" until it has been put through it's paces.  Just replacing the spring/follower does not guarantee a "certified" GTG magazine.  Only testing does that.  Mark such mags appropriately.

D&H from PSA for a quick bump, great mags/cost.

Then whore in the EE...

Colt
Okay
Brownells

Avoid anything C Products and Pro Mag.