Gotta have onion. A slice if grilled. If fried in a cast iron skillet (Anything else is sacrilege), I like to do it White Castle style and fry the burger with diced onion. And I second the Buttered, Grilled bun. Mine would have to be a Gluten free bun, but it is still damn good.
Grade A Wisconsin Cheddar
No bacon, and I love my burgers with Mayo.