I read Stephen Kotkin's excellent three part series on Stalin this summer and Kotkin (who is by no means a Soviet apologist; he's a conservative) thinks Stalin was a political genius, a modern Machiavelli. For example, in this interview [1] he explains how Stalin's administration was what he considers a "transcendental work of art" while also explaining how it was psychotic and evil.

I highly recommend Kotkin's books. You can jump into the second, Stalin: Waiting for Hitler, 1929-1941 if you want to learn more about him as a leader.

[1] https://www.youtube.com/watch?v=nz1ROc0pTU4