From the airport
Train: C1 to Madrid-Recoletos. It's a 27 minutes trip. If you are in the city for a few days buy a T-10, Multi-person card for 10 journeys, (€12,20). It's cheaper than a single ticket (1,50€).
Taxi: It's a 20-30 minutes trip and is around €30.
Aerobus shuttle: Drops off at Atocha, Cibeles (night) and O'Donnell-Doctor Esquerdo to Airport T1,T2 and T4 and costs 5€.
Subway: Take L8 from airport to Mar de Cristal. Change to the L4 and take to Serrano (approximately 40 min).