When we talk about to transportation the Subway is the better way, connecting most of the Capital Federal, this is the fastest way to explore the city.
The Subway is divided in several lines from A till E and two special routes that are H and P. The main line for me is the C line which goes through the main spots in the city, starting in Retiro Station and it end in Constitucion Station. This way to travel is actually well use by the locals too, the main problems with traffics jams, strokes and parking spaces makes this only option to ensure they get to work on time.
A line: from San Pedrito to Plaza de Mayo
B line: from Juan Manuel de Rosas to Alem
C line: from Retiro to Constitución
D line: from Congreso de Tucumán to Catedral
E line: from Plaza de los Virreyes to Bolivar
H line: from Hospitales to Facultad de Derecho
P line: (Pre metro): from Intendente Saguier to Centro Civico or Intendente Saguier to General Savio.
During the rush hour is kind of heavy, annoying and brutal sometimes, the frequency is low for the many people using it so from Monday to Friday around 7am till 9am and from 5pm till 8pm will be the worst hour to ride so if you are close to your destination and you have the time, enjoy a nice walk.
Paying the ride is another thing to consider, the rates are increasing constantly and the pay method is sometimes kind of hard to get, the SUBE card or MONEDERO card are the ones you need but there is a chance to buy it in cash. Almost all of this lines goes to 5.30am till 10pm, check the Buenos Aires Subway webpage to be sure you are on time.
Be aware like in every country of pickpockets, they usually are there and is not rare that this happens, even more during rush hours. Another tip to be prepare is when you must change lines, this could be hard, I got lost a few times, get the right platforms could take you more that expected, the signs are not clear sometimes and get the other side is like walking through a maze.
Subway Pic from Buenos Aires Ciudad.