A sweet or pleasant natural smell is a FRAGRANCE, PERFUME, or SCENT:
I love the delicate fragrance of roses.
The perfume of the flowers filled the room.
She smelled the sweet scent of the lilies.
ODOUR is a slightly more formal word meaning ’smell’ and is often used when the smell is unpleasant:
There was the unmistakable odour of sweaty feet.
The word STENCH and the informal words REEK and STINK can all be used when a smell is very unpleasant:
the stench of rotting fish
The noisy engines filled the air with the reek of gasoline.
There was a bad stink in the kitchen.
If a smell just lasts for a short time, carried on a current of air, you could use the word WHIFF:
He leaned towards me and I got a whiff of garlic.
BOUQUET is the characteristic smell of a wine or liqueur: This wine has a rich, oaky bouquet.