Mayonnaise is the best condiment because it is not really a condiment.
It acts as a base to accentuate the flavor of other things, or to nullify acidic/metallic taste of certain dishes. It's amazing on fries and fish because of its vinegar content.
You can add anything to Mayonnaise, that is the brilliance of it.
If you add simple vegetables you create tartar sauce, and most non-oil/vinegar/mustard salad dressings use mayonnaise as a base.
You can add hot-sauce, ketchup, mustard, horseradish, vinegar, ect.
The only thing mayonnaise does not work with are foods that require a thin citrus or marinade base "Like some form of steak-cut for example", that's eaten separately.
Ketchup, in of itself, is not very remarkable. It goes well on fries/chips, hot-dogs "which are specifically american hotdogs" and eggs. It works well with fried foods that are too heavy for mayonnaise and mustard "like fried chicken or fish". However, you can mix mayonnaise with ketchup to make "fry sauce", which is the premier sauce to use on hamburgers, chicken-burgers, and fish-burgers.
Ketchup with horseradish creates cocktail-sauce.
Ketchup, mayonnaise, mustard, horseradish, vinegar, and hot-sauce, makes my own personal favorite "do everything" sauce which is just heaven.