I'd have to say McDonald's... I mean they're not that expensive and they're everywhere, in fact there's one right by my school.
I know it's not good for you... but it is so good! Haha... anyways I have watched Super Size me but that doesn't stop me from eating it :P Watch this, it's about a guy who loves McDonald's! haha
For healthier fast food (oxymoron... although an oxymoron is only supposed to be 2 words...) I'd probably say Subway. There is one right by my school too. Although I know it still can't be all that healthy.
Mine is Taco Bueno. I'm not sure if they have any locations outside of Texas, but the food is awesome! It's kinda like Taco Bell, but they cook most of the food in-store, and it tastes like real food, unlike Taco Bell.
I watched a video on what happens when you only eat Mcdonald's for a week, the dude gained a lot of wieght, and was addicted to it. That is the main reason i don't eat fast food, because it slows you down and can be a depressent.