Estimates as to the number of Soviet war dead vary somewhat but are generally counted in the tens of millions. That is, the invasion resulted in 20-30 million excess deaths. To put this in perspective, this was from a pre-war Soviet population of approximately 165 million.

The material damage was hardly less severe. Infrastructure in the occupied areas was devastated and as late as the 1950s some western Soviet cities lay in ruins. European Russia had been home to the majority of pre-war Soviet industry and, despite the heroic evacuation of industry in 1941, much of this fell under German occupation. (Examples: some 63% of coal and 58% of steel production was occupied by the Germans and largely lost.) Overall Soviet GNP and industrial stock fell by a third during the invasion; that which was left was largely redirected to the war effort, bringing the Soviet economy to the brink of outright collapse in 1942.

So the economic damage was immense. At the very least, the Soviet economy lost a decade of progress. Industrial production in 1950 had just about reached 1940 levels… itself an impressive achievement in recovery. This is not to touch on the degree to what the defence burden detracted from industrial growth, pre- and post-war. Nor did the invasion drastically alter the structure or nature of the Soviet Union: nuances aside, the Stalinist elite forcibly restored economic/political pre-war norms and means of control.

Which is why I'm wondering how, in the face of the above, does anyone argue that the German invasion was “the best thing that ever happened to them”?

With the exception of the diplomatic sphere – which saw the extension of Soviet influence into Central Europe and international institutions – it’s hard to see how the war sparked any improvement in Soviet fortunes. A significantly “better thing” to have happened would be the uninterrupted economic progress that would have resulted from Germany not invading.

Sources: Harrison’s USSR and Total War and The Soviet Union After 1945 are particularly relevant. For the broader picture see Alec Nove’s Economic History of the USSR.

Tsarist Russian and then the Soviet Union and now contemporary Russia have been a great power in the world for centuries. So I'm not sure your question as asked can be answered. The USA was the only major power in WWII that did not sustain civilian infrastructure damage.