As the overall quality of beef has taken a severe downward spiral over the past decade, the one remaining sure thing is the ice cream. Quality remains high, and the flavors generally delicious.
My current favorite of the major chains is Persicco. http://www.persicco.com/ - particularly the dulce de leche granizado.
I used to frequent Nonna Bianca (a single shop in San Telmo) but have not been in some time. They just never seem that happy to see you. http://www.welcomesantelmo.com/san-telmo-guia/xk5bjn7p8m/Nonna-Bianca In addition to the couple dozen regular flavors that you see pretty much everywhere, they also make a variety of unusual flavors.
Freddo, the "big player" is off my radar. I'll eat it, but I don't buy it. On my first few trips (mid to late 90s), it was incredible. After several years of not visiting Argentina, I returned (2006-ish), raving about the ice cream, went to Freddo and was sorely disappointed. (It had been purchased by some conglomerate.) It apparently has improved some since the big quality downfall.
I'm curious to hear the thoughts of others, including little shops that put out a great product.