National Museum of Art of Catalunya (MNAC)
During the trip I did with my friends not long ago, we were told to go and see the museum mostly for the views. Once up there, before even entering the museum the panoramic view is incredible, like a sea of buildings :)
We went in the afternoon and the fountains were turned off, at night they are turned on and are illuminated and the atmosphere is much more of a daydream than during the rest of the day (but it is anyway very beautiful). While you're climbing up, the museum progressively becomes bigger, more imposing, lovely :)