Could anyone please give some tips on cost efficient, 24/7 transportation in Barcelona, going from the airport to Las Ramblas, and back? Also, in general, what is the best way to get around the city? (e.g. Las Ramblas to barcelonetta, sagrada familia, etc)
The Aerobus willmtake you to Placa Catalunya at the top of Las Ramblas. It does not run 24/7 but does run from 5:30 am to a bit after midnight. Fare is 5,65 one way or 9,85 RT. Website is aerobusbcn.cat
I just did this last year. As a Spain Tourist newbie I thought it was exceptionally easy, and the personnel all helpful and friendly. You won't have any problem.
We also used the Aerobus as others recommended, it was quick, convenient and economical. To get around Barcelona, we purchased tickets for the HOHO bus and used it to get around to some places. We also used the metro. Taxis are reasonably priced for travel within the city but probably expensive to/from the airport which is about 30 mins from the Las Ramblas area.
Another vote for the Aerobus. I caught it early in the morning from Placa de Catalunya, close to my hotel. It felt safe and was quick. I was only in Barcelona for two days (all I needed), so I spent one day walking around Las Ramblas and environs and another day using an all-day transit pass (subway + buses) to get to farther-flung sites (highlight: Parc Guell).
We took the Aerobus too. From our flight we walked in the direction of ground transportation. I was considering using Renfe, because it was cheaper, but came across Aerobus first so went with them. It's very convenient, pay as you board the bus (with cash from an ATM in the airport). Placa Catalunya is the start of the Ramblas.