Answer:
The smell of a certain laundry detergent that I don't even know the name of. It reminds me of my childhood bestie, who was the bestest friend in the whole world and my first friend in America (I'm from Russia and came to the U.S. when I was 6 years old). Their house and her clothes always smelled of that detergent, and whenever I smell it, I still remember her and our friendship. I made a mistake that I won't mention, and we got separated, but my childhood best friend lives on in my heart. Along with that detergent.