Tex-Mex is judged by the cheese enchilada and crispy beef taco combo - so I would have to go with Maudie's. Jorge's isn't bad either. After that, go to east Austin.
FSM is an incredible restaurant, even though their head chef left / is in the process of leaving. It's definitely not Tex-Mex, it's very similar stuff to what Rick Bayless cooks on his show. I tend to save it for special events though, especially the Sunday buffet, which is extremely pricey.