I've been to Barcelona twice, each time for about 4 days. Not long enough. I continue to promise myself that I will go back and stay for at least a week. It's a beautiful and exciting city, user friendly with much to do. Avoiding tourist season is definitely a must for the big sights like Sagrada Familia.
We stayed at the Majestic Hotel on the Passeig de Gracia - great location and quite lovely hotel. They have a super rooftop bar with wonderful views. I was super happy there. The time I visited before, I stayed at the Hotel Pulitzer which is just down the avenue closer to the Placa Catalunya. Great hotel and much more reasonably priced, but still very high level service and atmosphere. I would probably stay at the Pulitzer again over the Majestic, though both are hard to beat.
Highlights of Barcelona were the Picasso Museum and Montjuic with the Miro Foundation and the lovely fountains.