The lamb cheeseburger at La Bonne Soupe on 55th Street in New York City is right up there. It's topped with Gruyere cheese and served with excellent fries.
If we're talking beef burgers, I really like The Counter in Times Square again in New York City. It's really reasonably priced for New York and you can pick from a ton of toppings. I did a third pound burger last time with blue cheese, au jus and cucumbers and it was phenomenal. Plus, their Parmesan fries are great.
You can see the burgers my friends had in the background and the toppings on theirs.