"The best" are the new "enhanced" H&K mags, but they are not currently maketed to civillians in the US . Write your Senators so we can buy them next October.
Currently USGI 30 rounders with Wolff springs and green followers are near the top, some would give the nod to the same mag made by LaBelle and coated in black teflon. If you are buying USGI mags, make sure they have 5 weld spots down the back, not 3. There were a bunch (either Sanchez or Cooper) that had 3 and did not feed 100%.
USGI 20 rounders with the original aluminum followrers are great, too.
I like US (not Canadian)Thermolds, but some say the lips may crack if you leave them loaded 10 years, or that they explode if you drop a full mag on a concrete floor (I do neither regularly).
Prices on preban mags have been falling, and supplies are loosening up. There seem to be a lot of LaBells around, I guess people have stopped "hoarding".