Mine is lilacs, because there was a lilac bush at the bus stop that I used to wait at with my mom when I was a kid, it was my school bus stop and the public bus stop so we spent a lot of time on that corner and now when I smell lilacs I just immediately feel like a kid again.
When I was little my mom had an electric cookie press that made jet-puffed cookies. Why it was electric, and had to be plugged in, I have no idea. It was almost exclusively used to make Christmas cookies (trees, presents, snowmen, etc.) and the smell of the dough and the finished cookies was so unmistakable and unique that I cannot describe it exactly but can immediately recognize it decades later.
Another smell is the combination of coffee brewing and a classic American breakfast of bacon, eggs, and toast being made by my grandparents. The kind of smell that instantly wakes you up and makes you stumble down the stairs half awake. I don’t ever drink coffee but the combined smell is intoxicating and always reminds me of them.