...on what?

This matters.

Wisely

Fuck mayo, marry mustard, kill ketchup.

Mustard. Get that other shit out of here.

Mix them.

Equal parts mayo and mustard, about half that of ketchup, and you got a delicious fry sauce.

What are we eating?

Wisely

Mustard. It's more versatile. Besides the obvious condiment uses, it's also a useful ingredient in cooking, can alleviate muscle cramps (eaten), helps with bee or wasp stings (topical), and can be used as a dye.

Not Or. And.

Fries with a mustard mayo mix, or a mayo ketchup mix, or a ketchup mustard mix, or a mix of all 3, are all fantastic.

When I do burgers at home, I do a mustard mayo mix on the bottom bun (followed by lettuce and tomato slices), and a ketchup on the top bun.

All 3 three bring something to the table. It the variety and proportions that give you so much flexibility in creating amazing food.

Mayo.

Horseradish.

Mayo+ketchup (fry sauce) for fries. Ketchup+mustard for hot dogs. All three for burgers.

Depends what it's on, they're completely non-interchangeable.

Grilled sausage in the forest: mustard.

On my pasta earlier this week: ketchup.

On my sandwich this morning: Mayo.

There is a time and place for everything.

On what?

Depends entirely on what I'm putting it on

Ketchup is out from the start. Which leaves mayonnaise and mustard. Which, in turn, depend on the occasion.

Belgian fries with mustard would be as bad as bratwurst with mayonnaise...

Hot sauce.

Can I eliminate one from existence? If yes destroy mayo

For steak/pork and fries - béarnaise

Desert island? Mayo. I mean it's damned near a complete food.

Mustard is the only one I feel like I would miss. Mayo is rarely needed anywhere greek yoghurt wouldn't work and ketchup isn't my cup of tea.

Mayo, mayo, mayo, mayo. I'd eat mayo flavored icecream if only the frozen dairy industry weren't so cowardly

If you're wondering who puts mayo on a hotdog, the answer is "probably one of your brothers," according to Clint Barton.

I put ketchup on everything.

Eggs, sausage, grilled cheese, fries, hash browns (pretty much every type of potatoes except baked or mashed), cheeseburgers, baked beans, hot dogs, corn dogs.

There's more I just can't think of them atm.

I fuckin love ketchup and will die on the ketchup hill

Purely condiments, ketchup please. I never use the other two. As cooking ingredients through... Mustard is indispensable. Overall I use a lot more mustard (seed, wholegrain, and Dijon) than any condiment.

For a murder weapon? Ketchup, I'd freeze it into a knife and then escape while everyone thinks the blood is just more ketchup.

Mayo + mustard

All of these have more variety and nuance then you might think, at least based on what major US grocers carry.

Mustard comes in a ton of varieties, from smooth to course to whole-grain, or with extra flavors like horseradish or cayenne or champagne. I've even heard of bourbon and curry mustards.

Ketchup too is an entire category and not beholden to tomatoes. You can use all kinds of fruits like mango or golden berries as your base, add sauteed garlic and onions, then blend it up with some brown sugar and vinegar for a mighty fine condiment.

And homemade mayonnaise is just on another level. It's pretty simple to make, but it doesn't stay fresh long. Otherwise try the Polish Winiary brand, it's 90% as good, especially on something like sweet potato fries. Mayo is also the basis for aoli and nearly every picnic/potluck "salad" (e.g. potato salad, broccoli bacon salad, etc). Stop buying the Kraft and Hellman's crap, those are mostly flavorless soybean oil.

What I'm eating will determine which two of those I am combining together. Potentially all three, in some cases. But there are also times when all the of those are better left out, in favor of barbecue sauce.

Rarely/never ketchup. I only eat that stuff if a friend or someone already put it on a plate/dish being served to me without asking first.

Mayo & mustard are both awesome depending on what they're being put on. And sometimes they team up & can be used together e.g. they work well together in tuna.

PS - You should throw butter in the competition, a lot of people would vote for that over mayo :)

Mayo, duh