I hate to just restrict this to one, since I have no number one. So I will break this and go with a few.
The smell of a summer or warm morning, but the downside to it is that it has to get so damn hot when it happens. I hate that.
The smell of rain, or any moist or water environment. Very nostalgic. Well everything that is smell related is nostalgic almost.
The smell of chimney smoke during a winter night when I am riding my bike home.
The smell of boiling vegetables, it inspires creativity since it takes me back to 1997. Hard to explain why.
The smell of a professionally cleaned and repainted apartment. Too bad the smell wears off after about two weeks.
Used to be the smell of tar (in small doses), since it reminds me of my good days as a kid.
You would love to live my life. I hate that nasty smell to the point were I am even thinking about writing a poem about it. I will end up doing it though.