Last year Cooks Illustrated had an article about how to make the best burger. The author experimented with many techniques and the end result involved including butter in the ground beef and freezing it so it would hold its form better during cooking. You can check the online article & recipe.
There are a few burger toppings I enjoy including avocado and grilled pineapple, but a good burger is all about the fundamentals of the beef, bun and the traditional lettuce, tomato, pickles and onion toppings. The bun needs to be soft but lightly buttered and toasted/grilled to add flavor, texture and structural integrity. The beef patty needs to be correctly proportioned with the bun, have balanced seasoning that does not overpower the natural flavor of the beef and needs to avoid too much oil/grease that results in the gut-bomb feeling. Added cheddar cheese is welcomed and almost a necessity to ensure you are comparing apples to apples as it is hard to compare a hamburger from restaurant A to a cheeseburger from restaurant B.