No fast food will be particularly healthy, as most fast foods use cheap processed carbs which are really rather awful. That's if we're taking "true" fast food. Anything that keeps away from processed foods (basically carbs) will be fine. Jacket potato and beans is a good "fast food" - but clearly not if waiting for it to be prepared from scratch.
I keep away from any mass produced breads and processed and refined carbs as doing so you might as well just eat a KFC or McD's (makes me feel sick thinking about it).
Best fast food for me is popping into a shop/deli which has roasted chickens ready to eat - and buy one and nom away
But I am an ogre.