Porto is the gastro heaven to me, I love Mediterranean cuisine anyway. Here comes a review on a restaurant in the center, strolling the streets in Ribera quarter at night, than finally eating out in the harbor at locals' place.
First evening I was looking for a nice place for dinner in the center and I finally found Camafeu on Tripadvisor. It's a small, cozy restaurant on the first floor overlooking the square. As soon as you enter the place, you feel the welcoming atmosphere, service is excellent and top quality. The design is authentic and elegant with some rustic elements. Booking a table in advance is highly recommended!