Russia had the greatest impact.
Britain and the USA had a huge impact also, but the Russians under Stalin would still be fighting the Germans in Vladavostok-the benefit of a dictatorship with a motivated population.
Britain taken out in 1940 would have meant the Germans might have even gotten to Vladivostok without that second front to worry about, Roosevelt could have stayed out of the war.
The USA was the world's arsenal and kept everybody going with trucks, ammo, aircraft etc. With them out of the picture, the Russians would still have won, but it would have taken longer, been bloodier, and the Iron Curtain would been in Wales or Ireland, not Germany.