Lezama- old school porteno
http://www.guiaoleo.com.ar/detail.php?ID=300
1880- across the street- also extremely old school. my wife says Lezama is better, and she is never wrong, but I like 1880 too.
http://www.guiaoleo.com.ar/detail.php?ID=1059
El Casal de Catalunya- go with a group, order the cocoinillo (roast piglet). its sinful. yep, its expensive. but worth it.
http://www.guiaoleo.com.ar/detail.php?ID=2579
Not in San Telmo- its actually more like Boedo- but up Cochabomba- is Martita.
I like it a lot- there is always a line. mushrooms are good, so is the squid, the pastas are homemade, natch.
Martita cooks, her daughter and husband serve. No tourists.
http://www.guiaoleo.com.ar/detail.php?ID=2133