If you arrive late at night on a low cost airline I recommend just taking a taxi. Arriving after 22:00 I have to charge 10€ more for someone to stay up late to give you the keys and public transport becomes slower and more complicated-especially with bags. During normal hours there is a shuttle bus to Plaza Catalunya and you can take a cab from there or check your phone map for the best route on public transportation. The bus routes change so any information can become outdated. You may try B-25.