You know the line, the Mark Twain line, about the coldest winter he ever spent was a summer in San Francisco. Well, you can add spring to that, at least this year. The last few weeks have had us all fantasizing about a quick trip to Mexico. Of course, we either can't afford it or our bosses won't let us go. But if we can't have the Mayan Riviera for a few days, we can at least have a damn good margarita, and the best place to get it is Tommy's. After a couple of owner Julio Bermejo's concoctions you won't care where you are anyway. Bermejo, a purist on the subject, stocks more than 200 brands of 100 percent blue agave tequila, distilled from the fermented sugars of the blue agave plant and nothing else. It's the widest selection in the United States, and one of these treasures (3-4-5 A?ejo, made from the saguaro cactus) costs $350 per shot. As you might expect, Tommy's house margarita is something special: Herradura Silver to begin with, fresh (but of course) lime juice, and no salt unless you ask for it (it detracts from the tones and textures of the tequila). Tommy's Heavenly Margarita Mix is now on the market, in case you'd like to try this at home, but then you'd miss out on the friendly atmosphere, the kitchen's Yucatecan specialties, and the opportunity to savor Chinaco Black Label, the rarest tequila on the planet.