There is no way I can choose ONE favorite place in Spain... so I'll just choose one of my favorite places.
Tarragona.
Tarragona is about an hour south of Barcelona and situated on the Mediterranean sea. People lived there hundreds of years before the Romans came and laid out the town in a pattern you can still see today. The Romans called it Tarraco. I see little bits of Rome all over Tarragona - in the streets, and the people. Tarragona is on a bluff overlooking the sea.
There is an impressive medieval cathedral in Tarragona as you can see above and also below.
There are numerous picturesque and quiet squares and cafes in which to enjoy them.
What I love more than anything though, is the sense of community in Tarragona. When you go out at night the cafes and restaurants are filled with locals from babies to well over 90 years. Like in Barcelona and other parts of Catalonia, the locals will all dance the Sardana dance together. It is a beautiful thing to see.
There are also the castellers, "human towers." Building a castell is another Catalan tradition with a vibrant casteller community in Tarragona. You can read more about the castellers
here. It is a thrilling thing, to be in a crowd of Tarragona locals watching the castellers.
Tarragona is a beautiful, vibrant and extraordinarily livable town. Just outside are several wine regions such as Montsant and Priorat, medieval hill towns and seaside villages, monasteries and natural wonders.
It is one of those places I would go back to time and time again, and will, next June.