Which animal in the world has the best sense of smell?

It depends on how you define 'best'. If it is for one particular chemical in the air, like a phermone, then the answer would be a moth. If it is blood in the water, then it would be a shark. But, if you mean best as having the largest capacity for the most complex chemicals, the answer would likely be a bear, specifically Polar Bears. Their sense of smell is about 7 times more powerful than a Bloodhound. A Polar Bear can smell a seal carcass from 100 miles away and can track multiple targets at once.
Dogs.