I would argue that the USSR is only the most responsible when looking at it through the loss of human life. It seems likely they would have never made it to Berlin without lend-lease. No one knows if the Germans could have regrouped and done better without the aid to the USSR, maybe the same number would have died, just in different locations. But, most of the USSR's ability to move material around was due to the allies supplying them with logistics vehicles, both trucks, and railroads/engines/RR cars.