Ranch. It's an American staple condiment. There are plenty of completely homemade recipes, but I find this one is particularly easy, and I always have these ingredients on hand.
There are quite a few vegan mayonnaise's on the market these days. Typically I will use the Just Mayo or Follow Your Heart brand. Both have good flavor and consistency. You can also make your own mayonnaise, and I have a recipe for this in my cookbook Simply Fresh, found on page 103, for those that have it.