I will always recommend El Sombrero to anyone looking for good Mexican food in Calgary. We always eat well and love the authentic feel of the place. The live musician is always a treat as well.
It's not perfect (a bit too loud and the menu could have more vegetarian options) but it's pretty darn good.
I suggest you order the guacamole appetizer, the house special, and a glass of sangria. That'll make for a great evening out!