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.