Best AK Design?
Certainly as a new member, I yield to those with more experience or exposure to the guns out their, but I think the question should be a little more specific, as in best at what? Accuracy, durability, reliability, or parts interchangability?
The PKM is a great belt fed AK variant, and interestingly enough, when studying the design, is the AK-47 "upside down". Its durable, accurate, reliable, and can meet or exceed performance from some of the best .30 caliber belt feds in the world.
As for the opposite spectrum; the light recoil, high r.o.f. and controllability and concealability of the Russian AK 74 SU makes it quite a competitor. Whether the job is close range suppression fire or taking out targets at close range while clearing rooms, it is definitely a contender in 5.45 X 39.5 mm. With a couple of 40 round RPD mags tapped together, it offers a lot firepower in a very compact package for close range encounters, and can still be a menace at moderate ranges.
Parts interchangability and reliability are again a big plus compared to other "precision constructed" platforms.
Again, only my opinion, since neither gun has ever left me "dissappointed"", I am partial to both. If you have never shot either or both, in both modes of fire, I would urge you to try them before you comment.
I agree there are better quality guns, newer technology, and guns capable of delivering the same or better performance in a "sterile" environment. In battlefield conditions when your gun going "bang" can mean a lot, I would trust either of these before I would some of the "polymer" wonder guns that are in the process of being evaluated.
There is also a reason that I didn't include the 7.62 x 39 mm AK47 variations, of which I have fired a number. The issue I have is not with the guns as much as the caliber. The 7.62 x 39 mm is overpowered for close range, and many a Vet owes his life to the "mag dumps" by those who field the gun and attempted accurate full auto fire at longer ranges. The problem was worsened when most manufacturers went away from the heavier milled receiver guns. Having shot Chicom AK-47's to Eastern Bloc AKM's, the caliber, in my opinion, is an issue. Its "over kill" for close range, and despite how well built the AK variation you consider to be the best, it is simply not a round designed to "reach out" and be effectively accurate, especially compared to rifle calibers past and present, such as 8mm, 7.62 x 54R, 30-06, 7.62 x 51, etc.
Again, only my opinions based on my limited experience!