For a quick burger I love In-N-Out which has no locations in CO. This burger was an excellent substitute, very tasty and generous in size. Plenty of toppings available to make any kind of burger you might desire. Also had an order of Frings (onion rings and fires) and found the rings to be of excellent quality and the serving was more than enough for 4 people. With drinks count on at least 8$/person, so it is not cheap but the quality is worth it.
Pros: Great burger, clean 50's style atmosphere