Visited Barcelona for this first time this past June as part of our honeymoon, and thank to the advice on this board had the BEST tapas (and restaurant in general) experience of our trip at Paco Meralgo. We went for dinner early (between 8pm and 9pm) before the crowds usually got there so never had the wait. The smoking and non-smoking halves of the restaurant separated by the chef's area was perfect for us as non-smokers (a rarity in Europe). We were so enthralled with the quality of the tapas and the unrushed experience that we went back a second time during our short 3day stay in Barcelona just to try a host of other items on the menu. Almost everything was so good we don't even have a favourite. The only wrinkle in our experience was a single large staple we found in the baked scallops dish which was served in clam shells - we brought it up with the server just to let him know but he just laughed it off (so make sure you watch before you eat!).
Anyways, on another note, don't make the same mistake we did and expect to go to Cacao Sampaka for their amazing Azteca hot chocolate afterwards for dessert, because they will likely be closed already - rather you will have to go to Cacao Sampaka pre-dinner, afternoon break , or breakfast.
Here is a link to the photo we took of the menu (as of our visit in June 2008) in case anyone wants a gander at the tapas menu.