I've always heard The Vortex was the best (I've got to try it sometime) but I'm pretty fond of Farm Burger in Decatur. You won't get better (fresher) meat and toppings.
I forgot about Farm Burger, they're excellent! Especially if you like unusual toppings.
Although I was not let down trying out a Vortex Double-bypass burger (that egg was AWESOME on there) or taking a few bites of my wife's Hell buger... The Fox Brothers BBQ take on a burger, with pulled pork, was something else! The smoked flavor stayed on my pallet for several hours. A decent atmosphere, it's no vortex, but the flavor is worth checking out.