If you find yourself in Madrid, you will not regret making time for the Prado. Whereas some museums, like the Hermitage, go for quantity, the Prado seems to pride itself on extreme curation. It did not feel like a huge museum (at least the tour of it that I did), but every single room had at least one true masterpiece.
I agree. Do also visit the Reina Sofia[1], mostly dedicated to (modern) Spanish art.
It's your chance to experience Picasso's Guernica[2] in real. To me it was one of the most moving and impressive, while also disturbing exhibits I've ever seen in any museum.
The following quote is perhaps telling, in it Azaña, (president of the Republic) tells Negrín (prime minister, overseeing the evacuation due to the civil war) that:
> The Prado Museum is more important for Spain than the Monarchy and the Republic altogether... consider the embarrassment if the paintings were missing or seriously damaged... you would need to shoot yourself.
Well worth a visit. Queue early, bring water in summer: its hot waiting outside.
If you do get in early, consider heading to the diagonally opposite top corner: its where a lot of really nice stuff is you'll want to linger over, and you can avoid a crush and then backtrack.
I was totally surprised by some gems in the collection. Pre-raphelite works. Don't miss rooms, give everything its due.
And the naval museum. It's really amazing and not so famous. It's 175 old and has the map of Juan de la Cosa, the earliest preserved map of the Americas. And since 2007 the museum has hosted a specimen of moon rock from the Apollo 17 mission.
Will happily concur with the many voices in this thread advocating a visit. The Prado is absolutely one of the world's great gallery experiences. Make sure to check out Hieronymus Bosch's triptych 'Garden of Earthly Delights' which they've thoughtfully placed a sofa in front of so you can really sit and enjoy it.
There is a restored copy of the Mona Lisa there, painted at the same time as the one in the louvre, from a slightly different angle. It’s great— hardly anyone is there to get in your way if you want to see it.