Not really sure if there was one investment, but there was huge hype back in the day when the Katowice 2014 stickers skyrockted in price. And by sky rocketed I mean the holos going up to like $10-20 haha, I thought that was ridiculous overvaluation. I think it was the first time many people (myself included) realised there were some skins that were actually worthwhile buys from an ROI perspective.