Good topic.
Ketchup on burgers, unless you're going super high end (ie, black label burger). In that case you eat how they serve it to you.
Fries need ketchup. End of story.
Ketchup is essential to the pork roll, egg, and cheese. Salt, pepper, ketchup... yes please.
I also do ketchup on eggs and home fries at the diner. Never thought it was that weird until someone said, dude, that's weird. My wife does ketchup on diner grilled cheese. Still not sure what I see in her.
Mustard on hot dogs, every time. Spicey brown, none of that yellow crap. Same goes for soft pretzels. Dijon is good on sangwiches.
Here's a curve ball, mayo on a cheesesteak. With lettuce and tomato. Grease truck style, aka California Cheese Steak. Really good. Otherwise mayo only as an ingredient in potato salad, cole slaw, egg salad.
Bonus sauce, chili garlic sauce (the kind from the Asian section, jar with the green lid) is dope af on pizza.