Mustard barbecue sauce is (in my opinion) the best kind to put on barbecue. I'm not the biggest fan of most red tomato/ketchup based sauces because they can be so sweet or faux-smoked. Mustard barbecue sauce has much more range! You can have it with a nice vinegar bite or super spicy, or you can tone it down with brown sugar until it's almost more like a honey mustard.
I had my first taste of mustard barbecue sauce at Mark's Feed Store
in Louisville, KY about ten years ago. I have been addicted since. I tried my best to create something a little similar to theirs but I might just need to order a bottle. :D