The Soviet Union had the largest sub fleet in the world when Germany attacked in 1941. Over 200 boats of various types. But they were victims of Russia's rambling geography being divided between four widely separated fleets; the Baltic, Black, Northern and Pacific. Given the Soviet Union's obvious army-centric approach, they also seem to have been poorly maintained and to have lacked sonar or modern fire control equipment. Other than the sinking of the
Wilhelm Gustloff late in the war with heavy loss of life I can't think of too many Russian submarine successes that come to mind.
You can read about the
Wilhelm Gustloff disaster here:
http://www.wilhelmgustloff.com/