I used to not like any condiments at all, so whenever I ordered a burger or hotdog, I'd get it plain. Chicken sandwich? Plain. Everything plain. My friends used to laugh at me.
I did finally learn to like mayo -- but only Kraft. I'm not too thrilled with the more vinegary mayos like Helmann's. And definitely not Miracle Whip. If I want a bit of a kick, I'll add some wasabi to my mayo.
On my meatloaf, I'll add tomato soup, sauce, or juice -- never ketchup. Ketchup... blech.
I voted mayo but I'll also use ketchup. The only thing I ever put mustard on is a soft pretzel and it has to be spicy. My DD goes through a bottle of ketchup a week its her only condiment. The other DD won't even touch the mayo container she hates it.
since i am allergic to tomatoes, ketchup is out of the question. I eat mustard on just about everything. Honey mustard, hot mustard. if not that I will eat ranch.