At Home: Spicy mustard and onions, or just spicy mustard if I'm feeling lazy.
In Chicago: Chicago Style!
But my new favorite...
In Boston:
Speed's Hot Dogs! It's a half pound knockwurst, marinated in apple cider and brown sugar, grilled over charcoal, served on a grilled bun, topped with a perfect combination of mustards, chili, a special sauce, relish, onions, and more.