If we had one close to me, I would definitely say Chick Fil A or Raising Canes. We sadly do not have either of those awesome chicken establishments near me. Due to that tragedy, I would have to say my favorite fast food place is Wendy's.
Wendy's used to have the best regular chicken sandwich that I can remember. I would get it with just mayo on it and it was pretty close to fantastic. Sadly they changed their process or recipe or something and the sandwich isn't quite the same as it used to be. It is still good, but not the same.
There is a greasy spoon burger type place a couple hours North of where I live. They have some of the best burgers I have ever tasted. Like I said though, they are a couple hours away from where I live. Wendy's is the closest fast food I have ever tasted to those burgers, so that is another reason why Wendy's is my favorite. There is something about the double stack that is just the perfect combination of meat, cheese, pickle, onion, ketchup, and mustard. It is hard to find the words to describe just how awesome it is.
Finally, the clincher for all of it... The Frosty. Need I say more!