Tough question because it is a matter of degrees. Some dogs like bulldogs have poor senses of smell compared to bloodhounds. Rats have excellent senses of smell, perhaps as good as a bloodhounds adjusting for the size difference between the species. But since both species have similar olfactory nerves, the bloodhound would have to be the 'winner' due to the surface areas inside the nose cavities (maybe 50 times larger than a rats). I have read dozens of veterinary reports on this type of subject and most are inconclusive. The title of 'world most sensitive sense of smell' goes to a certain specie of moth, that can reportedly smell a few molecules of a female moths hormones from miles away. The problem is that the testing equipment used for the test is not nearly that accurate. I would suspect that the same is true in the case of the rat vs. the bloodhound.