Modernista architecture
The modernista facades are the most photographed subject in the city, which is precisely why the obvious shot is the hardest one to make interesting.
Sagrada Familia
Skip the plaza opposite, which everyone uses, and work the side streets two blocks out, where the towers rise over rooftops and drying laundry. After dark the lit facade against a deep blue sky is the strongest frame in Barcelona.
The Illa of the Discord
One block of Passeig de Gràcia holding three competing facades. Go early on a Sunday before the tour groups arrive, and use the opposite pavement for a straight-on elevation.
Palau de la Música facade
A dense corner of mosaic, sculpture and glass on a narrow street in Sant Pere, best lit in the later afternoon when the sun finally gets down into the canyon.