When people visit Barcelona for the first time, they will be mostly directed first to the Gothic Quarter, the Gaudi monuments and the Cathedral (Santa Eulalia) of Barcelona. My favorite Cathedral though is Santa Maria del Mar. Why? Go inside and see what I mean: this cathedral impresses with its structures, not with overloaden decor. Elegant and sober Gothic style. Santa Maria del Mar represents the Gothic Catalan architecture.
If you want to save on entrance fee, go during times when a mass is held. But make sure you respect the ceremony. There is a possibility to visit the rooftop of the cathedral aswell. Find information on the webpage: http://www.santamariadelmarbarcelona.org/tours/
Located in the Born District, the closest Metro station would be L4 (yellow line) Placa St. Jaume. Walk down the street Argenteria and you end up right in front of the cathedral.