...are not, in my opinion, in Santiago; they are in Valparaiso and Santa Cruz.
Here in Valparaiso, Pastis (corner of Subida Concepcion and Abtao) is, quite simply, fabulous. It has, without doubt, the finest duck breast (with duck-fat fried potatoes) I've had anywhere. The quiche starters are fantastic and the gazpacho is also really very fine (or so I've been told, as I don't like gazpacho myself). The desserts are pretty decent but not as good as the savoury food.
Pastis is expensive by Chilean standards (including a cocktail, 3 courses and a bottle a wine to share, it comes to around 20-25,000 pesos per person) but for the quality of the food and the restaurant aesthetics, it's at least two or three times less than something similar in New York, Paris, London or any western city would set you back. The wine list is decent and well-priced.
Seema (an old friend from home on a recent visit) modelling duck breast, duck fat fried potatoes and salad)
Down in Santa Cruz, you'll find the best Peruvian restaurant in Chile (and I really don't say that lightly), La Casita de Barreales. Absolutely everything on the menu is fantastic, in particular the ceviche (of which I'm not usually a huge fan). The pisco sours are brain-murderingly good. And the other great thing about dining there is that you're just a short trip away from all the vineyards in the Colchagua Valley.
Both restaurants are better than any I ate at over three years in Buenos Aires by quite some distance.