The Subte, short for Subterraneo (underground) is the metro system in Buenos Aires. It's the easiest form of public transportation for tourists. You don't need a transport card (the SUBE card) and can pay by putting your credit card or digital wallet/phone right onto the new turnstiles to pay.
Line D is the most convenient line, starting in Plaza de Mayo and passing through all the main sites: the obelisco, Recoleta, and Palermo.