When i go back home to visit, i always pay a visit to Palermo Soho.The shops i go to are Lupe, De La Ostia, Sal si puedes, Seco, Mishka, Jazmin Chebar, Maria Cher (the last two are in shopping malls aswell), Felix and Sixfeet sell nice clothes more men, especially sixfeet t-shirts (for my partner)
Then if you are looking for something vintage, retro, etc. you have the 5ta. Avenida Gallery in Santa Fe Avenue (between Libertad and Talcahuano) basically there are shops where they sell second hand clothes,shoes and accessories.There are few shops that sell brand new clothe too.Then 5 minutes walk away from this gallery, Marcelo T. de Alvear 1441, there's Juan Perez which also sells second hand clothes but a little bit more high end (if it make sense ,you can find original Valentino coats from the 70's and stuff) for high prices.
There's also the Bond Street Gallery in Santa Fe Avenue (between Montevideo and Rodriguez Peña) there's tatoo shops, a few record shops (if still exist) and some clothes shops like Fujimoda and stuff,there's also a couple of shops that sells clothe for men ( i have to say most of the clothes sold there it's teenagery looking but that's my opinion).
Santa Fe Avenue has plenty of shops all along it (from Santa fe and Carlos Pellegrini to Santa fe and Coronel Diaz )
For shopping malls the best is Galerias Pacifico, google it and you'll see what i'm talking about, Paseo Alcorta and Alto Palermo sell nearly the same brands so...Patio Bullrich is more high end but you still find some of the brands you'll find in the last two mentioned shopping malls.
Hope this helps