The most memorable experiences in my travels are invariably the local street food vendors. In Buenos Aires, there are two places that are great for this - Costanera Norte, and Costanera Sur.
In picking a street vendor in Buenos Aires, the key thing to look for is the range of condiments available! This is such a great advantage over restaurant eating where you get only one or two choices, out in the street they lay out all the options for you to see, and pick out. There can be multiple variations on the chimichurri sauce, the standard red one, one with ají picante (excellent, and quite spicy), one a la provenzal (green parsley and garlic, maybe spicy or not). There will be mustard, mayo, ketchup and salsa criolla (onions, tomatoes). Never in a restaurant will you get all this, even in street vendors, not all street vendors serve all of these options, but it is worth walking around to find one with the range you need.
As for meat, it is of course parilla style, grilled meat. Bondiola (pork), hamburger, and the best of all - choripan is available. The paty bread for hamburger was never very good, texture and quality is poor. The choripan or bondiola uses pan for the sandwich, somewhat similar to large french bread loaf and much better tasting, though can make for messy eating since it is a bit tough. Add the other oily condiments, and maybe the cheese which also is invariably oily in Buenos Aires, and this is really messy eating, but it is all worth it. The taste, the experience, is something that will be cherished for a long time.
Costanera Norte
Take a cab, or a colectivo. From Microcentro take the colectivo #45 that has the Ciudad Universitaria sign to the river bank across from the Aeroparque airport. Costanera Norte is the northern part of the street that runs along the coast, along the river bank in Buenos Aires. It has many attractions, the local government site Ciudad de Buenos Aires - Costanera Norte lists the areas (and construction status!). There are many big restaurants and clubs in this area, but there are also nice green spots to enjoy the scenery and eat good street food. One such spot is the Plaza Puerto Argentino, around 1km north of the Club de Pescadores on Av. Costanera Rafael Obligado.
The picture shows a somewhat fancy street vendor - large permanent place, and covered space for eating, named Kotoroko's. These guys had a spicy provenzal sauce that was fantastic. And the choripan "full" came with ham, fried egg, cheese, lettuce, tomato, and excellent spicy chorizo sausage. Great food!
This area is also much less crowded than Costanera Sur. The only downside is the relentless, loud noise of the traffic on the road that carries seven lanes of constant traffic, and the once in a while noise of a plane landing or taking off. Still, the scenery is worth a visit.
Costanera Sur
The Reserva Ecológica Costanera Sur is a very famous tourist spot, so there can be large crowds here on holidays and weekends. They have numerous food vendors on the street here, and at least one serves up a large number of condiments (but no cheese). The local goverment web site has a lot of information - Ciudad de Buenos Aires - Reserva Ecológica Costanera Sur.
The south entrance of the reserve is the main entry and activity spot. From Microcentro, take a cab to get here, no colectivo goes close to the street. But with a 10 minute walk, colectivo # 111 goes on the adjacent street A M de Justo. The cross street is Vera Peñaloza, which is close to Ave Independencia or Estados Unidos. The Ecological Reserve is huge, and a walk around its trails is anywhere from 2-5 km for a round trip. On holidays, there are people renting bikes if needed. There is also a large park in this area.
Since this place can be crowded, a line of 6-8 people in front of a street food vendor means a 20-30 minute wait.
At all these places, Choripan is an excellent choice. Bondiola also is quite good, but the meat is tougher so that makes for even messier eats! Get the pan sandwich instead of the paty sandwich bread for any meat you choose - it takes much better. And then lay on the sauces - as less as possible, since the bread does not hold well to the liquid. Some locals seem to eat just the meat and the bread. This is the best way to taste the choripan, so have two - one loaded, and one with minimal amount of salsa.