My favorite is the Russian plum magazines. They were made by both Izhmash and Tula, but you're only likely to find Izzy ones for reasonable prices. I can't really say why I like them more than the Bulgarian mags, it's purely subjective.
After that I'd definitely go with Bulgarian. Circle 10/21/25 magazines are all made to the same specification. Generally speaking, surplus Circle 10 magazines are only seen in black. Apparently there was a plum model as well, it's exceedingly rare. K-Var also has new production 10s in OD Green and their plum color (which is quite different from the surplus plum color). 21s are seen most commonly in black, but also in shades of dark red and dark brown. K-Var (Arsenal) also imports 45-round Code 10 magazines in black.
I've also used Bakelite magazines of East German origin and they work well. While they definitely look great, particularly on a rifle with wooden furniture, I'd go with Russian or Bulgarian polymer magazines first. The polymer magazines are more durable, but that isn't to say the Bakelite magazines are weak.
I don't have any experience with Russian Bakelite (at least not in 5.45) and I don't have any experience with Polish polymer magazines or the steel magazines that are available.
More information can be found here. It's not a completely exhaustive list of every 5.45x39.5mm AK magazine ever made, but it's a pretty through breakdown of any that you're likely to see in the United States.
I bought these when Cope's had them for $20 a pop last year. New in-the-wrap Polish Tantal AK-74 mags. Also bought a few of the steel ones in the same condition:
Ha ha I got you guys beat on part of the great cheap mag hunt.
Back in 1998 when I got my first 5.45 Century Arms was selling NEW East German Bakelites (4) in the teardrop pouch WITH stripper clips, and a cleaning kit on the side for $19.95.
The second one I got had the stripper clips hidden inside the pouch but no cleaning kit for $19.95
Then I got 3 black Bulgarian Circle 10 mags for $9.99 each.
I have the 11 mags for the SLR105R and the SGL31 and the 5 rounder Romanian steel and 10 rounder Bulgarian black plastic I found along the way for bench shooting-these mags will outlast me. Communists know how to make mags-I give them that.
Fortier did a test on all AK mags in Shotgun News and the East German Bakelites came out on top in all his testing.
I have 17 magazines for my SGL31 not counting the 10-round magazine it came with. I'll probably end up with around 30 for it. I enjoy collecting magazines, I just wish AK mags were so expensive. That said, they are built to last.