I see this "California" board has a lot of San Diego action in it, so as a native with much time logged in local restaurants I'll share my favorites. These are places I take out-of-towners, and I admit a conscious bias against restaurants that put the view and "experience" above the quality of the food.
Not listed in order of quality.
Breakfast: The Hash House A Go Go, Hillcrest. Huge upscaling of midwestern favorites; from scrambles and hashes to the sausage gravy pot-pie to the Snickers flapjack that's bigger than your head. Very busy on weekends, but well worth the effort -- and you won't need to eat for the rest of the day.
Mexican: For quick food, you can grab good to excellent fast-mex in any part of town. For sit-down food, Old Town is the obvious choice. If you're stuck with people who can't think outside the taco, there's no better touristy-mex than Casa de Bandini. It has the look and menu of most old town spots, but the food is decidedly improved. If you want something more authentic, with dishes from regions of Mexico and the best Mole in the city, Old town can serve you as well at El Agave. 300 tequilas doesn't hurt the impression, either.
Italian: Bell'agio in Tierrasanta, on Clairemont mesa. Yes, it's in a strip mall. And at one point it was a pizza-hut (those trapezoid windows!). But this is an elegant, modern family-owned Italian restaurant that's head and shoulders above most every Trattoria in the gaslamp. Excellent pasta, seafood specials, and in-house desserts.
Pacific Rim: Roppongi, La Jolla. I'm not enamored of their entrees, but you can make an outstanding meal out of a few selections from their extensive asian tapas menu. I recommend the polynesian crab stack, the kefir cheese with flatbread, fried tofu, and the ahi poke as must-eats. And save room for the tahitian bananas dessert (almond brittle, bananas, caramel, grand marnier, and vanilla gelato). Little secret: Happy hour from 4-6 7 days a week, all tapas are half price. You can feed eight hungry chowhounds for $90.
Sandwiches: La Torta, with locations in La Mesa and Pacific Beach. Monstrous huge sandwiches, fresh torta rolls, delicious meats. In a world where chipotle sauce is becoming the new ketchup, theirs was early and still a standard-bearer. Oh, and they're cheap.
Seafood: Blue Point, in the gaslamp. The best of the Cohn group's generally great collection of restaurants. You'll be paying extra for the fine ambiance and excellent service, but you won't regret it once your food arrives. Outstanding crabcakes, perfect scallops.