I have always answered this question with....
puppy breath
Even when I was a cat/bird person.
So it still tops the list. 2nd favorite smell would have to be the smell of the ocean.
But it does make me wonder...what is it about puppy breath that people like so much? Most people think I'm silly when I answer that, but some people go nuts over puppy breath as well. I dunno...maybe it's an endorphin thing