Two of the most requested restaurants in the country are Frontera Grill and Topolobampo - Rick Bayless' places on Clark Street. You practically have to make reservations a year ahead of time to eat there, but you absolutely won't find more palatable Mexican food.
I prefer Frontera Grill personally, but Topolobampo is also wonderful - I just have simple tastes when it comes to Mexican so Frontera works for me.
Start with the bacon and garlic chive guacamole - you'll be ruined for anything else. Follow up with the lamb shoulder in black mole with extra cilantro, the mole poblano enchiladas, the fresh corn tamales with chicken (get extra carmelized onions), or the steak trio tacos - you're talking about an evening of pure enjoyment. I usuallygo by late in the evening, but if you are sucessful in getting a great table during prime time - you can finish your evening by taking a walk around the neighborhood - one of the best in the city. I'm usually leaving so late, I share a cab with one of the cooks - or the hostess on duty.
