The VIEW.
I took postcard quality pictures from the balcony of this apartment with a DSLR camera.
It's clean, has an excellent host that will point out all of the places worth visiting, and is comfortable. We were a family of 7 and all slept great. It has a few grocery stores nearby, and a huge 13-story department store (El Corte Inglés) about a mile away.
It's a short walk from the river near the Ponte Luis bridge that will take you to into Porto proper.