The bear has the best sense of smell in the animal kingdom, thanks to its superlative number of smell receptors. Because smell is a sense that is based on perception, it's difficult to pinpoint the animal that objectively qualifies as the world's best animal smeller. However, it is possible to infer how well an animal may smell based on the complexity of its nasal anatomy, particularly with respect to the number of smell receptors an animal has.
In practice, the bear does indeed seem to have an excellent sense of smell. Bears have been known to travel miles in a single, unbroken straight line in order to track down prey. Some bears, such as polar bears, use their sense of smell to overcome physical barriers to scent, such as multiple feet of ice.