They are all good unless bubbarized.
Original design 1949 Russian is a heritage and it looks gorgeous with birch stock. Later they change to plywood (politely called laminated) and finished production in 1958. Overall quality is of sledge hammer so to speak. They all were in service, so don't fall into "original" or "not used trap.
Yugo are the worst milled quality and they are overpriced for no reason. Well, they have grenade launcher. Duh.
Chinese have 3 periods: copy of the Soviet model, perfecting Soviet model and perfected Chinese SKS that is stamped and pinned instead of milled and threaded but you can get it newer, say, 1994 model. This may not be the best collectable item but they are the best quality and best shooters. Also never painted, well adjusted with smoothly moving parts, yummy!
Chinese SKS has other models, include import, custom and mix and match refurbished weirdos.
There are also Romanian and ugly Albanian. Controversial German Karabiner S, probably North Korean, may be something else.