Stalin stalled Hitler...

And Hitler hilted in the (Russian) snow.

As did Napoleon.

I don't know how much Stalin himself had to do with it. He refused to believe that the Germans were preparing to attack... and when they did invade, he spent the first week sulking instead of leading his country.

As usual, the Russian soldier's best friend was his rifle, not his high command.

You do know that nearly all invaders haven't been able to invade Russia, and I don't think that this shows that they have bad comanders

This depended very much on the period and the political context.

In the Napoleonic War, the Tsar was militarily inexperienced; and after the defeats of Jena and Auerstadt followed by the absolute disaster of Friedland, Tsar Alexander had the good sense to leave the direction of the battle to experienced officers such as Kutuzov and Bagration.

In the First World War, the situation was worse in that many of the high military positions went to court favorites rather than competent military men. To give just one example, the disaster at Tannenberg during the opening days of the war is directly attributable to the gross incompetence and personal vendetta of Samsonov and Rennenkampf.

During the 1941 Barbarossa campaign, the situation was perhaps even worse for Russia. The Officer's Purge had taken place only a few years earlier, with the victims (executed by the NKVD) included 3 of the 5 Field Marshals, 13 of the 15 Army commanders, 8 of the 9 Admirals, 154 out of the 186 Divisional commanders, all 16 of the Army Commissars, and 25 out of the 28 Corps Commissars. These murdered officers had been replaced by junior men who Stalin considered to be "politically safe", which meant that many of them were militarily incompetent and totally unsafe. It was this gang of newly promoted "party men" who were directing operations when Germany invaded.

Naturally, the fools and idiots were mostly weeded out during the apocalyptic battles of 1941 and early 1942... but by the time that competent military men were back in control of operations, the Russian casualties already numbered in the millions, and the bulk of the pre-war equipment had been lost. The defeats in the summer and fall of 1942 were due more to shortages than to incompetence.

By 1943, the typical Russian field commanders were a match for their opponents.

I worked for two years as a Mentor at the Canadian Army's Combined Arms Tactics School.