We don't get the best fast food selection here in Western Pa, mostly just the major chains like Wendy's, McDonald's, and Burger King. I really wish we had some of the less common ones that have been mentioned such as Sonic, In-N-Out, or even White Castle.
That being said, I like the Junior Bacon Cheeseburger from Wendy's. I always get it with ketchup instead of mayo, and since it's only a dollar, you can't go wrong with it.
White Castle cheeseburgers are also awesome, and I really wish I had a White Castle closer to me, still, if I ever see a White Castle in my travels, I always make a point to stop there, it's good stuff. I've had the microwaved ones that Solid mentioned, but they really don't do it for me, I don't like microwaved cheeseburgers very much, plus you can't get that awesome hot sauce stuff that they have in the restaurant.
The Big Mac is great too.
And for fast food breakfasts, nothing can surpass the classic egg mcmuffin for me, I enjoy that more than any other fast food meal.