I guess whatever is blooming and fragrant at the time
I have a NoID Cymbidium which is filling the greenhouse with fragrance just now, really love it. I thought at first it was a Den kingianum that was fragrant in there as they can fill the greenhouse with scent even when just a couple of flowers are open, but that dropped it's flowers and the scent remains and I've realized it's the Cym this time.
Onc Sharry Baby "Sweet Fragrance" is probably my favorite though.