Of course, it's not super terribly hard to be Pittsburgh's best Mexican food, as there are so few Mexican restaurants in town. Still, coming from a person who moved to Pittsburgh from places that had TONS of Mexican restaurants, the city is lucky to have Rey Azteca as it's better than any of the numerous Mexican restaurants I used to eat at on the West Coast.
From their awesome Burritos Mexicanos to their super good nachos and fajitas, Rey Azteca hasn't served me a bad dish yet, which is rare since I usually only like a couple things at restaurants. Even their pork burrito covered in green tamadillo sauce is great, and I've never liked anything with green sauce on it before.
Rey Azteca's only real problem is that they're kind of set off the road a bit too much, otherwise it's the best Mexican food in town, better than El Campesino, which isn't bad in it's own right.