The Barajas Metro Line (number 8) takes only fifteen minutes to deposit passengers in the centre of Madrid's financial district (Nuevos Ministerios). Taxis are available from the airport, but be sure not to accept offers made inside the terminals.
Updates
If arriving at terminal 4 you need a free bus transfer to terminal 2 to get the metro. If you do the same to take a taxi you avoid the horrendous queue at 4 and save yourself about 7 euros on the fare.
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