Of all Russian leaders, Stalin was the greatest administrator. By far.
Paranoid, bloodthirsty, ruthless, genocidal, power-hungry and traitor to the revolution, sure. But that guy knew how to delegate and how to organize.
If your task is to argue he is the best leader you should probably focus on this as trying to deny he was a monster is a losing battle. Look at his rise to power as general secretary, a post previously thought to be just secretarial (pun intended) but which he used to gradually consolidate power within the party. Look at his various economic plans such as the 5-year plan, his implementation of the GULAG system, restructuring of the Red Army, roadbuilding, canal building, factory development, various economic plans (such as the 5-year plan), the relocation of entire factory complexes to the East of the Urals in the face of the rapid German advance. Also look at the amount of incredible talent that he was able to foster. Molotov is arguably one of the most successful foreign ministers of all time. Zhukov, Rokossovsky, Vasilevsky and Timoshenko could all individually stake a reasonable claim to be the greatest general of all time and the fact they all emerged in that era under that leader may not be a coincidence.
Stalin's genius was ultimately organizational. Your argument should close by looking at the state of the USSR at the time of his rise to power and compare it to the state of the USSR at his death. No single nation in history (except maybe Genghis Khan's empire) has so quickly grown in strength over the tenure of one person's leadership. By this metric it's impossible to argue any other Russian leader as greater than him, not even Muscovy under Ivan the Terrible.