Stalin was one of the great winners of WWII and controlled one of the largest empires in history stretching from Germany to the Pacific. Churchill on the other hand was responsible for the collapse of the British empire; he couldn't even get elected again in 1945/1946. Stalin was the leader of the USSR for 28 years. Churchill was Prime Minister of Britain for 5 years.
The Soviets, long forgotten in the West for WWII, has recently been on the receiving end of a surge in revisionism in their favor (largely due to anti US sentiment). While the Soviets bore the brunt of casualties in the war, this figure disproportionately reflects their relatively mixed success. Without either the US or Soviets, WWII wouldn't have been won, but without Britain, the US loses its foothold in Eruope, and the Soviets are alone against Germany. Churchill's influence on Roosevelt meant a large amount of monetary support and, later, military support against Germany, and may have led to the US not attacking Japan first.