Thyssen-Bornemisza National Museum - Madrid, Spain
We visited the Thyssen-Bornemisza National Museum on a Sunday when we were in Madrid. Although there were many people, the Museum is large enough that we didn't feel crowded. We loved the assortment of paintings in this Museum and enjoyed it way more than the Prado - where there is no video because photos are not allowed at the Prado. If you are low on time and can only go to one museum, I highly recommend visiting the Thyssen-Bornemisza.
For more info check out their site: https://www.museothyssen.org/en