I consider Burger 21 to be a close competitor of Smashburger. However, I would say that Burger 21 is a bit more refined or less fast-foody than Smashburger. The burgers are very good and I like that Burger 21 has some unique "burgers" on their menu. Their shrimp burger is an interesting feature. I thought it would be whole shrimp sort of like a shrimp po'boy. Instead, it's ground shrimp in a patty form...with a lot of spice! Quite interesting. Be sure to brace yourself for the heat if you get it. Burger 21 also has awesome shakes. But you could get on serious calorie overload if you get a burger, fries, and shake. But what the heck, right?
Overall, I like Burger 21. Keep it on your short list for a more refined, "faster food", burger.