I agree that you can find highly unhealthy choices in nearly every type of restaurant, but given the general regular categories, I'd say that American restaurants probably have the widest array of healthier choices available. Also typically the more expensive the restaurant, the healthier it would probably be, in general. It's just too hard to generalize based off of type. True chinese food is incredibly healthy, yet most chinese restaurants are 95% full of highly unheathy stuff, unless you go to someplace nice and pay $30 a plate, so it's almost impossible to categorize like this.
If I had to pick a general category, I would say that moderately nice family owned restaurants would typically be the best choice, but I know one of those that is a catfish/rib buffet, so, go figure, you just can't really do this. I'd say that any place where you're paying for the quality of the food rather than the quantity is going to be healthiest, but they're often more expensive, but even then, there's nothing keeping you from eating too much or making poor choices.