As far as actual fast-food goes, Sonic is probably on the better end. They offer a good selection of drinks. I like to add diet cherry to rootbeer for a fun, though sugary, treat. The cream pie shakes are also 'to kill for'.
Like all/most fastfood chains, you aren't going to be able to find any low-fat diet fare on the menu. That withstanding, it is a fairly decent menu. They serve breakfast all day, which is a major plus. My favorite thing is a cheese coney dog (basically a chili dog). This falls into the basic, but good, category.