There are currently 78 milongas in Buenos Aires. No one has time to attend them all. For the most part, young dancers don't attend milongas for seniors in the afternoon and evening. Seniors aren't interested in attending the night milongas where young people dance. There are milongas of all types.
I suggested milongas during your recent visit and gave you my thoughts about the list for your group tour. I did the same for John when he came for the first time five years ago. I know the milonga scene well enough to recommend milongas based on the dance level and age of the individual. I hear comments from people about the milongas that weren't right for them. If one has only three weeks to dance, it's best not to waste time trying out milongas.
John's description was accurate. He knows where to dance in the city center milongas. He rents an apartment walking distance to them all. These milongas open as early at 3:00 p.m. I attend the same milongas as John.
I'll be happy to help you when you come for a month or two so you can establish a daily milonga routine and feel a part of the local tango scene.
I'm probably a bit more of a hybrid (or something), as I like a variety of different milongas. I must confess, I really do like dancing to some of the tango bands they have in BsAs, (as most bands here in the US don't really focus on the "Golden age" style of music. They're more into fusing jazz with tango songs, which gets old quickly for me). So a lot of nights, we would go to a milonga where a band would be playing.
My most memorable milongas this past trip were: Nuevo Chique (some really nice embraces there, and where I had the honor of dancing with someone very special)

; Salon Canning (with La Orquesta Tipica La Juan D'Arienzo playing, and they were great); De Querusa (listed as a practica, but it sure seemed like a milonga); and at Club Gricel (some great performances that night, and some great dancing in the milonga).
I tried to talk more to people on this trip, and get their thoughts on various topics. Some of the Argentines were very surprised that there was tango in the US. On one of the nights at Salon Canning, a nice man who was at the table we were at (we were all the guests of one of the performers that night), was skeptical about tango in the US (as well as me being a DJ). He was (politely) quizzing me about tango music, until he was satisfied that I actually did know a something about it. Later, after drinking some bubbly, he admitted that we danced well, (although I have to admit, he was a better dancer than I was).
To be honest, I had great time at the milongas I went to (even though the styles were very different at some of them). People were friendly to us. However, I couldn't do every milonga I planned on doing, (I didn't make it back to Sunderland this time). In any case, it's a great lifestyle (while on vacation) when you can do a early milonga, go eat dinner at 10, then go to a late milonga. Also, I did get talked into going to a fancy (i.e. expensive) restaurant, Chila. If you're into overpriced gourmet food, it was very nice.
The bottom line, I felt very comfortable there in BsAs, and look forward to my next visit, whenever it will be.