The State Tretyakov Gallery
State Tretyakov Gallery together with the State Russian Museum are the best collections on Earth of Russian visual arts.
The State Russian Museum
Three of the most impressive collections that I know of:
Prado Museum
Fantastic collection, especially of spanish painters, including Diego Velasquez and JoaquĆn Sorolla who are among the best painters ever lived.
The Boston Trio:
Includes three separate museums in one urban area.
The Museum of Russian Art (Minneapolis, MN)
A small, privately owned collection of Russian Art, the only one in the United States that is open to general public. We are lucky to have it.