Taquería Díaz serves up the best tacos in Buenos Aires. They're authentic, legit, the real deal, and every other cliche you can think of. They have tacos and quesadillas, sometimes chilaquiles show up for the brunch menu. Keep an eye on their Instagram for specials. If the Birria tacos are on the menu (they make a rare appearance) then run, don't walk, to get a couple!
They also make excellent cocktails and margaritas. Most bars in Buenos Aires treat the margarita like a Brazilian caipirinha (muddling sugar on the limes) but Taqueria Diaz does it right.