I have several...
I have a bottle of hair tonic that my grandfather used. It's about 20 years old, but if I smell it I immediately feel like I'm in his presence again even after he's been passed away for 16 years.
The smell of Lysol immediately sickens me, as it reminds me of my childhood dog that died. One of my first real adult tasks was to help bury him, and Lysol was present at the time...the smell still turns my mind to sadness and feeling sick.
Airbag dust...only smelled that once but I hope I never have to again. I'd never mistake that smell for anything else, and it would remind me of the accident I was in when I was 18 years old.
Old car smell: any gearhead will know exactly what I mean by this, and they will appreciate it! One of the best smells in the world...forget new car smell, that's for amateurs.