El Nopal is quite possibly one of my favorite Mexican restaurants in St. Louis. The atmosphere is very casual, but full of energy from being packed much of the time. The service is typically excellent and fast. The salsa and fresh chips brought out as an appetizer are delicious, but try a little salsa and stop, since its heat comes on only after you've already eaten a bit, and the different salsa batches can vary quite a lot in heat. You get a huge amount of food for fairly cheap as well. Try the chimichangas, which are fantastic!