I prefer BBQ on hamburgers over ketchup.
Ketchup is only for fries.
There's already a tomato in the burger. Mustard may be acceptable (I hate mustard anyway) but BBQ sauce is always a must!
Depends on the burger. Personally I don't like raw tomato on burgers, so it's not there for me to begin with. Normally I go for either ketchup (god help you if you eat "catsup"), or a mayo/hot sauce combo. BBQ sauce on burgers is good at times, but again it's a rare thing and depends on the burger.
I don't like mustard on burgers or dogs, regular ol' yellow mustard is gross (to me) no matter what, and dijons and the like are preferably used sparingly in sauces and marinades/glazes and the like*.
*Edit: Or a good dijon honey-mustard on a chicken sandwich. That's some good ***there...in fact, I think I know what's for dinner tomorrow night with the remaining chicken in my fridge. Hurray for healthy FFXIAH-inspired menu planning!