It depends on the type of Mexican you're looking for.
If you're seeking the standard Tex-Mex/Americanized tacos, burritos, enchiladas, and the like, you're not really going to find them on property (not sure if La Tortuga at MK will be open for you, nor what it's menu is, so this may be an exception.)
Most of the Mexican, notably in, well, Mexico, is a bit more traditional authentic-based dishes and more upscale than the tacos and stuff.
As far as the "best", for me it was San Angel Inn last year. Ribeye Tacos and Sopa Azteca were very good. Of course, I wasn't that hungry and had a migraine starting up when I ate at Hacienda, so that certainly tainted my meal there. (I also ate at Cantina and it was very good, though the meat was a bit smoky for my taste).