...a clean rivers breeze on a sunny day in Western Vermont. Now, that may be a biased answer because it falls on the heels of spending one of the most beautiful afternoons possible floating down the Battenkill River in Vermont. When you are moving swiftly downstream, can close your eyes, listen to the wind on the leaves and the occasional bird in the trees, everything smells more intensely. And when the river is a clean one, with fast moving water, and you are in the midst of summer, it is the most beautiful smell in the world. Something about silence makes things smell more beautifully (or terribly if it is a bad smell). Maybe it's the fact that it's unnecessary to utilize other senses - one can focus entirely on the smell.
Bathory was right that there are other beautiful smells. There isn't one that brings a smile to my face with the same ease as truly fresh air. It is a beautiful thing and there are fewer and fewer places we can do that which really is a shame.
Some of the worst smells in the world? Water shoes after canoeing and not washing them once in over a year. Although it isn't the shoes fault, it's the mildew in the shoe and clearly it is my fault. That and old fifteen passenger vans, unwashed drunk people, and major highways. Not everything can be roses and puppies.