Webers mustard.
If you have never had it you have yet to have mustard.
But the real secret to a great burger is start with quality beef, use fresh toppings, a quality cheese and avoid the basic hamburger rolls. Use quality breads and or rolls for the burger. Grill it. make sure it chars a bit but never serve it past medium. If your guest asks for well done say sure. Then serve them a nicely chaired medium burger and they will love how much better you make burgers than the arsonist who usually makes their food.