For Mexican food, I tried Frida Kahlo in Belgrano once, and it was really excellent. It was not tex-mex, but authentic Mexican (except with a salsa that was watered down for Argentine tastes). There's also a sort of Tex-Mex place, new, in Caballito on Pedro Goyena called Mexico DF, and from the menu, I think they serve a mixture of authentic Mexican and Tex-Mex. I haven't got around to trying the place, but it is always full on the weekends. It might be worth a try as it was also recommended in La Nacion. As far as Chinese goes, I've yet to try something amazing. But I have tried some Korean places that I thought were pretty good.