I love a good burger and Lawrence is blessed with a number of good local burger places as well as a number of premium chains. But when I want the best fast food burger, I go to Wendy's.
Their Double Stack (currently 99 cents) is the perfect combination of flavors and textures for me. The beef is good, the onions and pickles provide some crunch and the condiments, while basic, compliment the overall burger.
I try to avoid fries, but I have tried their new more rustic cut, and they are quite good. I like that they are pre-seasoned with sea salt.
Service is usually fairly quick, if not particularly friendly. But at this price range, they provide a good combination of product and service.
I am not taking out of town guests here, but for a quick fast food burger I have yet to find a better one in my all too extensive experience.