Mexican Food in Colorado is pretty damn good, but most of the good places it is more traditional mexican food and not TexMex as most of you are used to. This time of year the chili verde will be great as it is the season to see the hatch chiles coming up from New Mexico.
Tamayo, La Loma, Blue Agave and others all have good mexican food near downtown.
If you are in the suburbs, Hacienda Colorado has several locations around the city.
If you are in Boulder or Fort Collins, Rio Grande is pretty good and great margaritas.