Zhukov certainly can't be a post-Lenin leader, he wasn't anywhere near senior or influential enough in the late 20s. Kalinin was the OTL de jure Head of State of the USSR until 1946, too. Molotov was an Old Bolshevik, but most of his power came from being intimately associated with Stalin's power bloc.
Of the list, I'd say Bukharin, Zinoviev or Kamenev are the most likely.