let me start my list:

High-end restaurants:
- AbAc Hotel (basically a very chic hotel and spa built around a 2* michelin restaurant). Look for their midday lunch+spa menus!
- Comerç 24. Fairly my favorite.
- Moo @ Omm Hotel. Menu designed by the Roca brothers (from El Celler de Can Roca, #4 hotel in the Sant Pellegrino list).

Low-end (but good) restaurants:
- Bestiari (a favorite among FT'rs)
- Paco Meralgo (itsaboutthejourney took me there, and it's amazing)
- Can Manel (best Paella in town)
- Inopia (Ferran Adria's tapas bar). Get ready to arrive at 7pm and get a table at 9pm. Very busy but totally worth it.
- El vaso de oro. Great tapas, very busy also.

Hidden spots:
- Go to the pakistanian bakery in C\Hospital (50-something). Have the chicken rolls and any of their pastries...
- The terrace of the Mares museum or the Textile museum. Great spots to hang out and great iced tea in both places. No entrance fee required.
- The rooftop of the gothic cathedral. Amazing views over the gothic district.
- 98% chocolate ice cream at the italian gelatto shop @ Plaza Revolucion, in Gracia district.
- The jazz lobby bar @ Hotel Casa Fuster. Great drinks and music.

And whatever you do, stay away from the so-called gay restaurants. Awful food, awful service, overpriced.