Shake Shack [Madison Square Park (at 23rd St. and Madison Ave.) 212-889-6600]
Your arteries will not thank you for going here, but your taste buds will. I would try the double Shack burger with that special Shack sauce (similar to every other delicious special sauce it is Russian dressing-y). Make sure to ask for your burgers rare or medium rare, because medium is the default. Also go for a milkshake.