Palau Guell is, in my view, a must-see Antoni Gaudi building in Barcelona. As it's still relatively lesser known than Casa Batlo and La Sagrada Familia I don't think it's as crowded, which makes for an enjoyable visit. And with its location not far from the Barri Gotic, it's a convenient add on to a walk around the old quarter of Barcelona.
Antoni Gaudi, probably the best known art-nouveau era architect in the world, has left Barcelona with a number of amazing edifices. La Sagrada...