The only thing you should do in a fast food restaurants is use their restrooms when traveling.
That's NOT food and no one should eat it.
Gross. All of it.

It has to be Yo Sushi - because sushi is nice, because their restaurants are, apparently, reasonably good places to work (unlike other fast food companies) and because they make sure that as much of their ingredients are sustainably produced/caught as possible.

Subway's food is, in my experience, delicious; however, they have a reputation for treating their franchisees and staff appallingly badly. McDonalds: no, just no - the food is disgusting, flavourless slop, the staff are treated extremely badly and the company is known for its numerous awful policies. KFC: I ate KFC food once, about 30 years ago. I haven't done since and won't, as it was probably the most disgusting thing I have ever eaten in my entire life.

The chains there aren't the same as what they are in the states. Your McDonalds & KFCs offered healthier options that aren't available here (I believe McDonalds there offered coronation chicken sandwiches that were pretty good). Probably my favorite wasn't on the list and that would be Pizza Express. The pizza's weren't too fat and over-loaded with toppings.

At least Subway is reasonably decent food.

I voted for 'other' because none of those seem like good options to me. I hope they are all held to higher standards in your country. in my country, many of them have meat that is lower quality than dog food.

greasy kebab

I'd say eat what you like, but keep the sugar and other sweeteners down to a minimum. That won't be easy with fast food.

Chic fil A. They can do no wrong. Except their religious ideals.