Mayonnaise is the best non-hot sauce condiment there is.
Especially on fries. My favourite thing about Europe was that restaurants everywhere gave you mayonnaise to go with your fries.
At a Denny's at around 2 a.m. a couple of decades ago, an old boyfriend turned me onto dipping fries in tartar sauce (which I assume is mostly mayo). I don't have many opportunities to indulge these days, but in the cafeteria on Fridays at my old job, they would often serve fish 'n' chips with a vat of self-service tartar sauce. And then it was Katie bar the door.