That’s probably hard to say, since there were different methods of training, and some countries didn’t have time for training (see Russia - here’s your gun, go shoot the Germans with it. And they were lucky if they got a gun).
Also, there are huge differences in the morale and beliefs, etc of various infantry.
But if you’re just wanting to discuss training (isolating it), then wouldn’t it be the Japanese? I don’t know, I’ve just seen some of the actual footage of their training sessions, and they were pretty intense.
As for worst training (I know, you didn’t ask), probably the Russians (at certain times) and western Europeans (I’ve seen some of their training footage too, and it’s not impressive)