Although the d'Orsay in Paris and the Capitoline museums in Rome are my favourites, the Rijksmuseum is in my top ten. I believe it's only recently reopened after a major renovation, and I'd give a lot to see The Night Watch "in person".
I've recently been trying to expand my knowledge of art without any money to travel to the great museums. These online museums are just priceless for armchair travellers, but I've also found that, for remembering schools of art, periods and techniques, I can use a free tool called Lexicon (lexicon.geknowm.com).
I type in an art term and Lexicon comes up with a definition and a set of related terms. Then I study them at my leisure and ask Lexicon to "Test my memory" of what I've studied and track my progress.
I now have a greatly expanded knowledge of what I'd like to see in museums â€“ and a better understanding pf why it's worth seeing them.