While in Buenos Aires for a short time, I booked a day long walking tour of the city. First, the assistance pre-trip - both to nail down the details for the tour, but also with other general questions (e.g. best way to get from the airport to the city), was amazing. Everyone was so friendly and helpful with all of my questions.
I was lucky enough to have Ceri (though, I met other guides who, I am sure, would have been top-notch as well) for my tour; he was great fun! He had a deep love of the city and it's people and culture, and of course, was very knowledgeable about everything we saw. Heck, he even followed up with me after the tour with some recommendations for Argentine movies (which, of course, were both quite entertaining).
In summary, I would absolutely recommend to family and friends, and if there was a rating above excellent, I'd select that one. Rest assured, this will be a good choice for you.