My personal favorite fast food restaurant is Orean's Vegetarian Drive-Thru in Pasadena, CA--probably because I'm a vegetarian, but their food is really good, too! My favorite thing to order is the Vegetarian Philly sandwich which is just like the real thing except made with a meat substitute, really delicious and filling!
Before I became a vegetarian, my favorite fast food place was Tommy's Chili Burgers which is a Los Angeles based chain. Although most people get their burgers, I was partial to their chili tamales. What makes them so popular is their chili, which is more like a spread than a traditional chili. But it is addictively delicious. It goes great on their fries, too! And they're very nice in that they hold the salt on request! It probably isn't healthy but the chili tamales and chili fries is perhaps the most addictive foods I've every had! Once I became a vegetarian, I was able to stop going for about 10 years until they had their 50th anniversary a few years ago. I went once for old-time sake and have gone back on occasion--it's the only place I've ordered food that wasn't vegetarian since I've become a vegetarian--it's that tasty!
In terms of national chains, I like Del Taco (located in the Western states and the Great Lake states area) a lot, because of the freshness of the ingredients, especially the beans. I do go to Taco Bell a great deal as well (especially since they added the triple-layer nachos to the value menu). I also like that they gladly substitue beans for the meat so that I can order many of the dishes there. I wish their fresco (healthy) dishes were tastier and more vegetarian-friendly though. It would also be nice if you could get a small salad with quality healthy lettuce like romaine or spinach there as well. But I guess I'm getting my hopes too high, lol!