Between Palermo, Recoleta and Puerto Madero, is one neighborhood considered nicer than the others? Or are they all roughly the same (other than Puerto Madero is newer buildings and highrises)?
Well Puerto Madero is easily the most expensive area. It's a newer area and mostly high-rises that are modern. It's also the safest area as they have their own police force there. The locals might argue that it's the "nicest area" but I personally find it has no real soul and too sterile. It doesn't feel like Buenos Aires to me. Also, monthly HOA fees are insane here! I have some friends that own really high end apartments here and some of them are paying over $1,000 US per month in condo fees alone! A few with 2 bedroom apartments are paying $600 US per months in HOA fees.
Recoleta doesn't have too many newer high rises. Older properties for the most part. Palermo Soho has a few high-rises but not as many as Palermo Hollywood. Soho is zoned so no more more high rises there for the most part. I really enjoy both Recoleta and Palermo. Recoleta is nicer and less gritty vs. Palermo Soho/Hollywood. In those areas you will see more graffiti vs. Recoleta which is a shame. I have a friend that owns a beautiful house in the heart of Soho and she has to repaint it every few months from vandals just spray painting. You don't see that problem in Recoleta in the nice areas.
A really creme of the crop area for me in Recoleta is the quadrant of Libertador/Libertad/Ayacucho/Quintana if you look on Google Maps. Really upscale and posh high end area. But all older more prestigious properties there. No newer high-rises but quality of life is super high.
In Soho I really love quadrant of Armenia/Honduras/Malabia/Guatemala or a few blocks outside that area. In Hollywood I like Guatemala/Bonpland/El Salvador/Juan B. Justo. I own properties in all of these areas and they are awesome areas and rentals do well in them.
Nice high-rises in Soho are really great as there is so many restaurants, cafes and shops in the area. Palermo Hollywood as well has TONS of restaurants. I guess it just depends on what is important for you. I wouldn't live in Puerto Madero because it's too far away and traffic can be a nightmare getting to/from there as they are working on construction in the area. I like how central Recoleta is but I don't think you can go wrong anywhere in Recoleta or Palermo. Palermo Chico (Barrio Parque) is also a really high end area. Not cheap but they have some newer high rises there as well. I actually personally prefer over near Soho or Hollywood just because I like a livelier area where I can walk out my door day or night and tons of places to eat and drink, etc.
Me personally I don't like San Telmo to live. Nice to visit but too gritty for me. I read a few posts where people said they live there and enjoy it but there is a reason it never really got as popular as Palermo or Recoleta. Still seedy at night in many areas. It's not to say you couldn't have a good life there but I just prefer more upscale areas vs. San Telmo.