Well, I wanted to get some answers in regard of who was the best leader FOR the Soviet Union.
Maybe you should have worded it that way then? And even then it's not clear. Are we talking what's best for the PEOPLE of the Soviet Union or what's best for the ENTITY that is the Soviet Union. If it's for the people, then the answer is clearly Gorbachev. If it's for the entity, then I don't konw.
Anyway, it certainly couldn't be Stalin, he killed 40 million Soviets. That wouldn't make him the best FOR the Soviet Union, entity or people. One would think Lenin would be a good choice, but he was just as big of a monster as was Stalin, he just wasn't as good at it.
I really can't see how it can't be Gorbachev unless you're only criteria for "best" is keeping the Soviet Union afloat.