Here for my fourth trip to Buenos Aires and this time am spending three months. My personal favorites are El Mundo on Maipu for Bife de Lomo and Pichi Huasi on Uriburu (1/4 block from Las Heras) for Bife de Chorizo. El Mundo has gotten a little pricy but has been consistent good steak since my first visit in 1993. Pichi Huasi is a small hidden gem - a neighborhood restaurant. Another rather good meal at Pichi Huasi is the Medallions de Lomo con Champignons - a SMALL bife de lomo in a demi-glace. It is small but so good.
I also agree with the recommendations for Yugo. Last night I ate there and had a Bife de Chorizo that was huge. The Flan Mixto was excellent to finish off a very good dinner.