If talking about Stalin, it probably has few or nothing to do with Communism.
Now your questions:
From a perspective that acknowledges a state as something legitimate and/or necessary:
1) and 2) Together: His doctrine greatly sped up the industrial and overall economic and technologica development of the USSR in many senses (although it quite hindered it in others, namely genetics and agriculture). Else, his "policies on Nationalites" helped to diminish nationalist outbursts and conflicts spawning from them. The impact of his policies on the USSR had global implications that are nearly obvious.
Focusing on impact outside USSR: Mainly, his policies contributed to the development of China.
Also, Stalin had an important role in the creation of the State of Israel as he actively supported its creation by providing military supplies, political support for Israel's creation and facilitating emigration of Soviet Jews to Israel early on.
3) By far and large, Hitler. There's no way to even reasonably compare them. In fact, probably the worst thing Stalin did was actually enabling Hitler by providing economic assistance to Nazi Germany prior to the WWII and shortly after it began, and still, it's far from what Hitler did.
4) KP, Mechanical Engineer.