I first visited Pushkin State Museum of Fine Arts which had a nice collection of European arts but then a shock was waiting for me: Some of the pieces in the 'museum' were in fact copies!!!
All art pieces had descriptions but they were all in Russian so I could not undertand which were original.
Then I had the opportunity to visit The State Tretyakov Gallery The National Museum of Russian Fine Arts which had an enormous number of Russian masterpieces,unfortunately due to time constraints I could only see the halls in one building.
I definitely recommend to visit both museums to the ones who are gonna visit Moscow ever.