Traditionally, Stalin has been considered the more murderous of the two dictators, and that wasn't really controversial. Some of Stalin's purges and famines were state-managed genocide (directed at specific races and classes).
Nowadays, with better access to Eastern European and ex-Soviet archives, we believe Hitler probably killed a few million more than Stalin. Regardless, discussing which is the worse butcher misses the point - they were both fucking psychopathic mad men. And trying to squeeze them into the lens of left-or right wing is also absurd since few think Soviet Russia or the Bolsheviks were 'communist' - in the same way nobody believes the German Democratic Republic was democratic.
http://www.nybooks.com/articles/2011/03/10/hitler-vs-stalin-who-killed-more/