The word "whiffiest" refers to something that has the strongest or most unpleasant odor. Its antonyms would be words that describe things with a pleasant, mild or no odor at all. These antonyms could include fragrant, sweet-smelling, fresh, clean-smelling, or even odorless. For example, a garden filled with fragrant flowers would be the antonym of a whiffiest dumpster. Similarly, a freshly washed room could be the antonym of the whiffiest locker room. In short, the antonyms of "whiffiest" are words that describe things with a pleasant or non-existent odor, making them all the more desirable.