Today I tried El Vaquero for lunch and was impressed. It's on Midlothian Turnpike at Turner Rd...same shopping center as Marshall's (but on the other side of the center). Authentic Mexican, clean, outstanding pork tacos and it is open for breakfast, lunch and dinner 7 days a week.
La Casita in Northside on Brook Rd.
It is slightly more California-style Mexican than your average Richmond restaurant.
There is no lard in the beans, less grease, less salt.
The original owners were from Mexico City with roots in Lebanon, and the menu is still pretty much the same.
Note, this is a suggestion for the best in Richmond, not for a really great Mexican restaurant. I don't think there is one in Richmond.