Among the most beautiful cafés in the world is Café Tortoni in Buenos Aires. It is inaugurated already 1858 by a French immigrant, and inspired by the late 19th century cultural atmosphere (see Fin de siécle). Wikipedia tells more.
The walls are decorated by paintings. The café hosts tango and jazz concerts. There are poetry nights. The guest list is full of well-known artists. The chocolate cake is sweet and the sparkling wine is cool. There are no reason to pass this place if you are going to be in Buenos Aires.