Turns out the smells that invoke the strongest memories for me, personally, are connected to my grandparents' house when I was 3 or 4 years old. Anytime I get these particular sniffs, I'm immediately and intoxicatingly transported back.
Bacon grease on cast iron.
Green walnuts. No smell on earth like that. I'm 4 again under my grandparents' walnut tree.
Some really old garages. My grampa was a gearhead.
Old-school commercial pop coolers and the pop bottle vending machines with the tall, narrow doors that swung outward. No smell on earth like them and the anticipation of the cold deliciousness that followed that smell. Heaven to me.