This is the correct answer, but not because it tastes great by itself, rather it's because it's the base of most other dips, spreads, sauses, and aiolis.
It's utilitarian as fuck and goes on or with everything. Think of every other condiment listed here and think what you usually use it on. Guarantee it's also got mayo or even has mayo in the condiment.
Mayo and mustard are definitely what came to mind for me. And if the two, mayo is far more useful. Sandwiches of every kind, it's the base for numerous spreads and dips, and it even goes great on stuff like sushi. You can mix it with so many things to make unique and interesting sauces. Eg, I use chipolte and garlic mayo for many sandwiches I make (and sandwich like things such as wraps). Sriracha mayo is a classic in sushi. It's usually the basis of a good fry sauce.
Secret aardvark. Look it up, let the reviews convince you, buy a bottle online, and reply in a week when you try it. You only need a small drop each bite, so the bottle will last. Its also fairly thick so you dont end up with a bunch of wasted sauce