Kuma's in Chicago is by far the best I have ever had. And I currently live in NYC where my favorites are Shake Shack, Burger Joint, and Island Burgers and Shakes.
In Sao Paulo (my hometown) I really like the Pic Burguer (hamburguer of picanha, the best cut of beef
http://en.wikipedia.org/wiki/Picanha) at The Fifties.