There can be no argument about the importance of Barcelona's architecture and its impact on modern art. Thousands of students come to Barcelona every year to study the maestros of modernism, especially Domenech i Montaner and Antoni Gaudí—the genius behind La Sagrada Familia and the Parc Güell, and considered one of the most important artists in history.
But Barcelona is not just Gaudí; the city is also home to works by Mies van Der Rohe, Zaha Hadid, Richard Rogers, Frank Gehry, Jean Nouvel or Arata Isozaki.
I've selected the following architectural classics with the assumption that this is your first time in Barcelona; if that's not the case, check out these buildings: Gas Natural (Miralles & Tagliabue); Modern Art Museum (Richard Meier); The Forum Building (Herzog & de Meuron) and the Hesperia Tower (Richard Rogers). Those are the best examples of modern architecture in the city.
This is the work of the modernist master Josep Puig i Cadafalch, and it's one of the vertices of the (imaginary) triangle formed by this building, the Casa Batlló and the Casa Lleó Morera. The building was a commissioned by a Catalan businessman and was completed in 1900. Some have described it as "urban gothic,” but whatever label you apply to it, it's very impressive masterpiece.
This is my favorite of Antoni Gaudí's works in Barcelona. The Casa Batlló was finished in 1907 and represents the best example of modernism from the beginning of the 20th century. The influence of the sea (some think Jules Verne's 20,000 Leagues under the Sea was an inspiration) is obvious in the façade that a famous Catalan critic called "an underwater cave." Please don’t miss the tour of the interiors.
This is another Gaudí masterpiece and the last one he made for civilian use. It’s at the top of the Passeig de Gracia and is also called Casa Milà after the couple who commissioned the building from Gaudí. The use of stone and iron and the distribution of weight in the building were revolutionary at the time (1910) and the result is an incredible piece of art that is used for concerts, exhibitions and all kinds of activities.
In my opinion, this is the most influential building in modern Barcelona. Locals used to call it "el supositorio" (the suppository) or even worse things, but it is now admired for its presence and versatility. Designed by Jean Nouvel and unveiled in 2005, it employs lights and glass in a way to allows the whole structure to change color, and it’s now one of the main attractions for architecture lovers—both visitors and locals.
Palau de la Música Catalana
This is one the most beautiful music auditoriums in the world. Strangely enough, it’s not very well known (maybe because it’s not in plain sight), but that means you can often visit it without being mobbed by huge crowds. It was designed by another modernist master, Lluis Domenech i Montaner, and finished in 1908. My recommendation is to get a ticket for a concert (any concert), and see (and hear!) the full experience for yourself.