My favourites are fresh Tarmac or a creosote'd fence on a hot summers day after a shower. Castrol R is a close second.
Also love the smell of Sunday dinner wafting down the street after a morning out on the bike- gets the old tum rumbling every time.
my missus insists that the smell of a baby is quite nice .....however i prefer the smell of freshly cut cedar wood or pine resin .....occasionaly the smell of the forest after a summers rain is quite nice....