The journey time from the airport to the city centre is around 30 minutes depending on the time of day, however all means of transport take more or less the same period of time.
Transfer by Aerobus. The Aerobus is a fast (35 minutes) and economical (5.90 EUR) shuttle bus that connects Barcelona - El Prat airport (Terminals 1 and 2) with the city centre of Barcelona (Place de Catalunya). The route includes 3 stops at the strategic points of Barcelona: Pl Espanya, Gran Via-Urgell, Pl Universitat. The Aerobus operates all year round without exception with a departure every 5 minutes. There are two Aerobus A1 and A2. The first one at the start of Terminal 1 and the second one at the start of Terminal 2. A distinction to remember especially for your return because only this N° (A1 or A2) distinguishes the two shuttles in the return direction. You can buy your ticket online.
Train. The train is very practical because it serves not only the centre of Barcelona but also most cityes in Catalonia! It's also the cheapest way to get to Barcelona (2 EUR per ticket). The train only departs are from the T2 terminal. If you arrive via Terminal T1 you will have to take the free shuttle bus which connects the two terminals: it is a green bus, easily recognizable. Renfe is the name of the Spanish national railway company. This indication must therefore be sought in Terminal T2. The train stops at only two stations in the center of Barcelona: Estació Sants and Passeig de Gràcia. The line serving the city centre is called R2 North line. You can buy the ticket at a vending machine or a ticket office at the train station.
Metro. Barcelona Airport's two T1 and T2 terminals are connected to the Barcelona metro network, which is very convenient for travellers. Line 9 (L9) does not go directly to downtown. For example, it is necessary to change at Torrassa to take the red line linking Universitat or Catalonia square. Change Collblanc to take the blue line towards Provença, Diagonal or Sagrada Familia. The two metro stations are located at the exit of each T1 and T2 terminal and are well indicated. The cost for the trip is 4.5 EUR.
Bus. You can reach Plaça Espanya by bus 46, which runs from the two terminals T1 and T2. It is a very economical (2.15 EUR) option to reach the centre of Barcelona from the airport. The bus stop 46 is located at the exit of terminal T1 to the left of the Aerobus stop. There are two bus stops 46 at terminal T2. They are located at the exit of terminals T2b and T2c. The company TMB -Transport Metropolitans de Barcelona - that manages the buses in Barcelona. So that's the name to look for to find your stop. There are also night busses: N16, N17, and N17.
Taxi. Taking a taxi to Barcelona from the airport is the simplest and fastest solution. It's not excessively expensive if you're at least two people. The taxi point is very well indicated at the exit of the two terminals T1 and T2. There are a lot of taxis there and the queue is very well organized, as is often the case in Barcelona. Look for black and yellow taxis. Tthe minimum fare for taxi service from the airport is 20 EUR (including all additional charges such as airport entry and exit fees of 3.10 EUR). This means that your trip will cost you at least 20 EUR even if you go to an address near the airport. Usually the ride to the city center would cost you around 30-35 EUR.