Of the American chains, I prefer: (1) Shake Shack; (2) In N Out; (3) Fuddrucker's; (4) Culver's.
Some really amazing burgers I've had:
* Marlowe, San Francisco (High-end restaurant)
* Garaje, San Francisco (No-nonsense grubhub). Garaje is probably my favorite casual restaurant in the country - food is delicious, but the lines can be loooooong at peak hours.
* Burger Joint, New York (appears to be permanently closed

)
* Burger Brothers, Baltimore, MD (complete hole in the wall dump...but the burger is first-class)
Sarah