If you want to pre-book a taxi (as suggested by GK) I would consider using Taxi Class.

http://www.taxiclassrent.com/english/

I like them because they only use Mercedes cars (minimum class E) and you can always pay by credit card (which is not standard in taxis in Barcelona).

I’m not sure if any of the taxi companies here will wait for you in the airport building itself. I generally walk outside to meet them.