Well the Soviets at their best were supplied with LL and had massive help from the Wallies via strategic bombing of Germany, which resulted in removal of the 75% of the LW fighters from the East plus 1/3rd of the military budget spent on air defense and deprived German artillery of ammo, economic warfare via blockade of Europe and buying up of critical raw materials from neutrals, threatening so much of Europe with invasion that nearly 50% of German divisions were not on the Eastern Front as of May 1944 rather in occupation/coastal defense duties, plus they were in combat in Italy and were bombing German oil to bits before Bagration even happened and killed the Luftwaffe in early 1944. The very best Soviet offensives came after the LW was then not a factor due to losses in the West and bombing of oil production, economic issues due to blockade and bombing of production/transport, use of major German resources in the West (Atlantic Wall, V-weapons programs that cost more than the Manhattan Project, strategic air defense), and the use of nearly half of German divisions on coastal defense duty, plus nearly the entire German navy being used outside the Eastern Front. The Soviets were able to clean up against a severely weakened foe from July 1943 on, as they shifted a huge part of their strength West in increasing amounts from that point on (really actually November 1942 with the Tunisia landings and subsequent loss of 42% of the Luftwaffe in 6 months in the Mediterranean, plus Panzerarmee Afrika). By the time the Soviets went on the strategic offensive in late 1942 the Germans weren't putting 2/3rds of their strength into the East anymore and overextendedthemselves in the East.