At the top of this list is the smell of citrus. Dogs have a disliking for lemons, oranges, grapefruit, etc., and anything that contains citrus will repel most dogs.
However, this does not have to apply to every dog out there. Some may even like it.
You can test your dog's liking by eating citrus fruit and bringing your fingers to your dog's nose. If they like it, they'll lick it, and if not, they'll back away.