We stayed at the Ayre Hotel Rosellon, which is just a block or two from the Sagrada Familia and from a metro stop. We were able to access all the sights we wanted to see easily, either on foot or via the metro system. The rooms were not very big, but big enough for me and my husband, and the bathroom was quite nice. They have a rooftop terrace that serves tapas and drinks in the evenings, and the view from there is wonderful, it looks directly at the Sagrada Familia.
Here's the website: http://www.ayrehoteles.com/hotel-rosellon/
(there should be a link at the upper right corner to change the language to English).