I'm a burger purist - lightly toasted then steamed bun, patty with the right fat content (and no additions, just salt and pepper) - not too lean - either smashed thin or a bit thicker and medium/medium rare, processed cheese (it really is the best on burgers, although swiss is nice too I guess), some kind of "special sauce" (even if it's just ketchup and mayo), diced raw white onion, dill pickle slices and lettuce.
Most places can't perfect the basics, but have some kind of truffle oil, bacon, foie gras (I'm exaggerating for effect a bit here) monstrosity on the menu to compensate.