the inside which I appreciate more than some of the ornamentation that it typical. Gaudi's use of light here is unparalleled in church architecture IMHO.
Contrasted with his Nativity facade which is extremely detailed.
St. Peter's is hard to argue with if you include all the artwork in addition to the architecture, but I prefer Sagrada Familia for architecture alone.
Sevilla was impressive just in it's enormity and the cools stuff it contains (like Christopher Columbus' remains)