As a kid I haven’t opened any booster boxes, but I did open booster packs once in a while. My two most memorable pulls as a kid were probably the Crystal Type Kingdra from the Aquapolis set, and a holo Jolteon from the Jungle set. The second one isn’t all that valuable or special, but I love the artwork and it’s still one of my favorite cards in my collection. It’s also the oldest holo that I pulled myself and still have after all these years.
Another memorable pull was earlier this year. I bought a Korean CP6 booster box just to get the Korean Base Artwork Pikachu common, but in the second to last pack I also pulled the Pikachu EX Secret Rare. Since that was the only Secret Rare I pulled from the box, I was very happy it was the Pikachu, so I didn’t had to look for it on eBay. 
Another pull worth mentioning perhaps was a holo Chansey in my very first (Base Set) pack, which I opened when I was 7 years old. But a few days later I traded it away for a non-Holo Vulpix, Pidgey and Pidgeotto, since I preferred more cards over holos at the time. 
More on that here in the “Your first Pokémon Experimence” thread.
Greetz,
Quuador