My two favorites that I've stayed in are the Santo Mauro in Madrid and the Blackstone in Chicago. The Santo Mauro is just a gorgeous property. People there were impressed we were staying there on a work trip as it's a great choice but not a cookie-cutter pick. The Palacio del Retiro in Madrid has better views being on the Parque del Retiro, and I got an upgrade to an amazing suite for my one-night stay there, but I still like the Santo Mauro better.
The Blackstone is awesome and so is the Spanish restaurant there. I also was lucky enough to be upgraded to a great suite there, and they had really great Chicago-specific decor and books.