I believe they call that an oxymoron.
But in answer to your question, there's no "healthiest" fast food restaurant. However, there are healthy options in the restaurants. At subway, for example, they have lighter sandwiches. McDonalds has salads and so on. The thing to do, would be to go on the website before hand. They have nutritional information for all of their food, so you can compare. If not, just eat at fast food restaurants as a treat and enjoy the fat.