Arby's. There is no doubt.
It's all good, don't make me choose! Here's two of my favorites that aren't standard menu items:
Popeye's Chicken: french fries dipped into the mashed potatoes
McDonald's: Southern Chicken Biscuit, add bacon, and dump one packet of honey and one packet of hot mustard on it. It's fucking crazy.
Edit: I've been asked what the burger in my profile pic is. That there is the Tom Sellick's Mustache from Lunchbox Laboratory, in the (not so) lovely Ballard neighborhood of Seattle, WA. It costs $15 and comes with no sides or drinks.
Not only is their food delicious, but they have some of the best service I've ever had, restaurant or otherwise. Plus, the one nearest to my house is SUPER fast. I love that place. Fuck it, I'm going there right now for breakfast.
Five Guys is damn amazing too, but there's only like 5 of them in Houston, and all sort of far from me. Worth the drive, though.