Already a few years ago I visited the magnificent Palau Guell. It was as if stepping into a decor for a fantasy movie. Whenever you are in Barcelona and the opportunity to visit the museum I recommend you do.
Some background info on the building :
Palau Güell was one of the first important commissions Antoni Gaudí received at the start of his career. Eusebi Güell (industrialist, politician and patron of the arts) wanted Gaudí to build him this peculiar urban palace as an extension of the family home on La Rambla. It was built between 1886 and 1890.