The only "fast food" type I've seen mentioned here that I've eaten are Fuddruckers and Five Guys. It's Five Guys for me, definitely. I'm anxious to try In-N-Out when I head out west later this summer.
As far as restaurants go, I do enjoy Hard Times burgers as NMS alluded to.
That said, the deep fried "Heart Attack Burger" from Mother's in Fed Hill takes it for me... I mean just look at this description:
Beer battered burger stuffed with cheddar cheese, then deep fried and served with lettuce, tomatoes, onions, and chipotle mayo.
BTW, I seriously didn't feel all that great for about 20-25 minutse after eating one!