The original of what is now 3 locations Casa Don Juan is a staple of traditional Mexican food and one of my favorite places to eat in Las Vegas. I think there are other really good Mexican places in town, but they can be inconsistent. I've always had quality experiences here and have yet to try something I don't like.
I really enjoy the Puerco En Chili Verde, the Super Burrito, Chicken Enchiladas, and Bistec Ranchero. But have tried most of the menu and everything has been great.
The price point is great too. For the amount of food it's hard to beat. The Puerco en Chili Verde is $12.99 and comes with a solid portion of pork onions and tomatoes in a spicy chili verde sauce, tortillas, a heaping side of rice and beans.
They often have a Groupon for two entrees and two margaritas which is clutch, because you definitely want a margarita. They are massive and delicious.