Hey- I'd say anywhere quiet, small and dark. Most of the restaurants here have the same food, so that really isn't a big deal. It's all about atmosphere. A lot of people like going to puerto madero, which isn't bad, but you usually pay more than you really should. you can get equally good food in a number of places, including calle chile in san telmo or calle san martin in retiro/microcentro. Good luck, and don't forget to top it all off with a good malbec!
Cheers!