The general consensus here and at the Mak.com message boards ranks the pistols in this order:
East German
Soviet Military
Bulgarian Military
Russian Commercial
Bulgarian Commercial
But then, there are the weird Maks. The Chinese military maks, the Simson-Suhl German Maks, the Miltex Special Edition Maks - all of them command a higher price premium for various reasons. All of the above pistols, except for the Chinese model, are considered better than the plain varieties with the Chinese fetching the big bucks for its collectable value.
In general, if you can get an East German or a Soviet Military mak for under $250, grab it as fast as you can! I passed up a primo Soviet model at $150 a few months ago and have been kicking myself ever since. Don't pay more than $200 for a Bulgarian military model (I paid half that for mine). The exotic maks, if you can find them at all, should be picked up for no more than $300. The Simson Suhl and Miltex pistols are especially good. The other models I don't know much about.
Be wary of the Russian Commercial models, though. While they're very good, their adjustable sights are irreplaceable and if you lose that tiny screw (known to happen), you're screwed. Of course, you can always buy a new slide with better sights installed, but that'll set you back another $160 or so.
In conclusion, any mak you can get will be great, but you might as well go for the rare and exotic pistols while you can still find them!
Relevant links:
http://www.makarov.com/ - source of most of my information
http://www.aimsurplus.com/ - where I got my Mak, they might get more soon
http://www.jcarmory.com/ - last I checked, unissued military Bulgarians with all the accessories go for $140 on that site
And the myriad of retailers advertising in Shotgun News.