The best Mexican restaurants I've eaten in have always been privately owned, never chains. I'm willing to bet that the top ten are privately owned as well, and possibly not well advertised.
I'm certain I've never eaten in one of the top ten in the nation--I live way too far north for that! I have, however, found very good food. One restaurant was an all-bilingual family staff with the kids running around for added authenticity. ;) The food was great and they even had "real" coke from Mexico--with real sugar instead of the corn syrup we add in the states. It was actually in a rougher neighborhood and quite out of our way, but the trip was worth it.
Honestly, you'd be better off nipping over to a decent non-Mexican restaurant with a semi-open kitchen and a decent percentage of Mexican cooks and ask what the best Mexican restaurants are in town. Believe me, they know! If you don't speak much Spanish, there's always at least one translator on every shift, even in Texas. They'll know the food quality and kitchen sanity from the Cook's Grapevine, too. If you ask in English, they may tell you a more English- and American-palate friendly choice, so make sure you specify if you want truly authentic, incredible Mexican cuisine and are willing to take on the language barrier to get it. A Mexican restaurant for Mexicans south of the mason-dixon may not be bilingual!
I've had the privilege of working with three "mexican kitchens" as they're politely termed in the food industry, and those guys have never steered me wrong. They're incredibly helpful with everything from where to find the best produce to great local restaurants and where to get salsa lessons.