It gets on my nerves when I see this discussion forum repeatedly giving Madrid second-tier status among Iberian cities worth spending time in, so I wanted to take a moment to point out that you shouldn't miss a stop at the Basilica de San Francisco Grande
https://www.esmadrid.com/en/tourist-information/san-francisco-el-grande
especially since tomorrow is the 45th anniversary of the assassination of the Francoist Prime Minister Luis Carrero Blanco -- who was blown up in his car in front of the church by Basque separatists, using explosives obtained from American military sources.
The regime used this event as an excuse for further reprisals against the Basque population, even though many many people hated this guy, whose only positive quality was that he attended mass obsessively, thereby providing rebels with a positive time and location to target.
Inside you'll find great portrait paintings by fantastic Spanish artists including Zurbaran, Goya, etc. and there is amazing decor and architecture, without the crowds at better-known sites. Be sure to check it out!