I was the weird kid in school that dipped fries in barbecue sauce while everyone just used ketchup but now I see that a lot of people use a variety of things for dipping their fries and it's often a point of pride.
The hubs likes "fry sauce" that uses ketchup, mayo, and a dash of pickle juice which is really common out west like in Utah but not really anywhere else I've found.
In the UK, a lot of people use malt vinegar (my personal favorite) or curry sauce (yum).
When we used to work in Alaska, a lot of locals like tartar sauce so fries regularly came with that instead of ketchup. I think it's a Pacific Northwest thing?
What's your region and what sauce do the locals prefer? What's your personal favorite?