Volgograd.period.
Call me bias, but Volgograd is the most beautful city for me. If you're into history, you must know that this city is one of the important city in Russia during WWII; scariest battle in history, Battle of Stalingrad .
Well other city like Moscow, Irkutsk, Samara and few more (need to look at my travelog to remember all that) is equally beautiful too. Each city has their own uniqueness.