I've spent the last 30 years eating Mexican food around the Valley, and I've eaten at all of the old places, the standard places, the places that people have been shot in front of an hour after I left, and sure, the gringo places. Restaurant Mexico stands alone. There are an undefined number of genres of Mexican food, but to put Restaurant Mexico on a "worst of" list is akin to calling the Virgin Mary "homely." You just don't do it. It's an institution of not only Mill Avenue but also Tempe itself -- an icon of the old, a survivor of the new, and a legacy for the times ahead.
Aside from that, Restaurant Mexico rocks. And it has for 25 years. You'll know it as soon as you have your first Clare burro, chicken mole, or the sopes that got knocked around unnecessarily. RM has never strayed from its dependable menu of queso fresco-topped dishes, homemade tortillas, fresh beans, honest salsa and just-out-of the fryer chips that have always come in the same size in the same bowl for the last 25 years. Guess what? If you ask the girls nicely, they'll bring you more.
And really, if you're hell bent on staying away from RM, a little mom and pop place still surviving on Mill Avenue despite the knocks-down, repaving, corporate invasions and the sagging business of everything else down there--which says to me that a place simply can't suck to live through that and has a loyal following that loves it every bit as much as I do--but it also means that there's one less person I have to wait behind.