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.
A strong one is mint from the mint plants that grew along the fence between my grade school playground and the train yard on the other side
More recently cedar and coconut are a couple that I enjoy just for the aroma