Excellent advice here.
Echoing:
1) CalPep. If you show up ~7pm and wait in line, you'll get right in. Not bad for the US-types who don't want to eat late. Perhaps a good place your first night in town, then you can settle in.
2) Commerc24 has been my best meal to date in BCN. Lunch is a relative steal.
3) Cinc Sentis also had a wonderful lunch menu.