As far as I understand, when Stalin got to lead the Politburo there was not a vote for leadership but it was whoever showed that they had the most power. But how can you quantify power? Did they find a way to measure power or was becoming the leader a very ambiguous assertion?
First of all, in USSR there was never such a position as "party leader". Moreover, Lenin was "Chairman of Council of People's Commissars"; Stalin was "General Secretary of Central Comittee"; and Khrushev was "First Secretary of Central Comittee". In 1964 Brezhnev got the position of "First Secretary", and reformed it into "General Secretary" only in 1966.
Next, Stalin became "General Secretary" in 1922. That is when Lenin was still alive, and Trotskiy was one of the unformal leaders. At that time Stalin was considered only as "junior ally" by Zinoviev and Kamenev in their struggle against Trotskiy. Naturally, the newly created position of "General Secretary" wasn't too much powerful. Technically, there was voting, but no one really thought that they were electing "new leader of the party and/or politburo".
Next, after Stalin's death (or more exactly, in his last hours of living), Malenkov, Beriya, Khrushev and others divided the supreme positions of the state among themselves. There was a joint session of Politburo (or more exactly "Presidium of Central Comittee") and Council of Ministers, but considering how quickly they did this job, we may be sure that "voting" was purely formal: everything was decided earlier.
And talking of later elections, we have to say that Politburo members always voted, but we are unable to deduce any usable info from official documents, as protocols could be rewritten. For example, Gorbachev's election must be "tough race", but officially they all supported him. So it's better to rely on memoirs and indirect evidence. Every case was quite different from others.