There are so many wonderful
restaurants that it is pretty hard to decide on which is #1, but (since you insist...) I will nominate Las Cazuelas (on north side of Basilio Badillo, out a ways beyond most of the restaurants on that street).
It is a long-established family restaurant serving mostly home-style (but elegant) Mexican food from old family recipes. Mama is a very friendly, courteous hostess and Papa is a funny, often ironically witty pleasure. They have a somewhat limited menu (supplemented by a few specials), but as is frequently the case, the small menu means that what they do they do very well. The setting is lovely; the east side wall has hanging plants (and, if I remember correctly, a waterfall). Not cheap by any means, but worth a visit for a leisurely, relaxed delightful meal.
Dave