While technically used in WW2, the SKS was...
You know, I looked into this with my highly.... well, limited, resources and I couldn't find a credible source that said the SKS was actually even in steel during any part of WWII. I found a few references to some rifle from Simonov, but no indication that it was actually an SKS prototype.
I'm not a professional historian, so I don't have the time/money to chase that rabbit down exhaustively, but I couldn't find anything about it.
I'd prefer an M1 Carbine, but the best was probably the M1 Garand. I think the M1 Carbine may have even been better than the Stg 44, simply because it was a little more mature design with a little more time (and other resources) for development. Definitely would say (in my completely subjective opinion) that the Garand was a better design than the German autoloaders, again, more mature design with more time for R&D.
Worst? Still going with a Mosin. I know we discussed this ad nauseum in the other thread, but...
All the SMGs were basically the same, except the Thompson, so I'm saying the Thompson was the worst. Sten, M3, all the Soviet SMGs, they all cost basically nothing and worked well enough.
Best pistol was probably the Hi-Power. I guess the Germans get that one. And Canada! Don't know of anyone else who used it extensively. Walther P38 was double action, so that's cool, but who actually likes DA/SA?