Ethereum, Netflix, Vanguard Lifestrategy 60 (all +40-50%). With eth and nflx I learned that the market is essentially random from my perspective. Vanguard gave me the habit of investing regularly and thinking long-term. I guess also a critical illness policy, taken out in my 20s - at the time I thought “Well, I’m young and immortal but ok why not” - which gave a later bad event a silver lining.
Best. And Worst.
1997: I worked at Football365. 18 months later, the company had IPOed and the gain was, er, life-changingly big. Like many others working at dotcoms back then I felt like a master of the universe. And then the stock market fell over horribly in March 2000. I ended up with about a 100% gain, but the drop in value from what it had been made it feel like the worst of losses. But I learned so much working there and made a career, so from the distance of 2018 it’s a win.
2008: After my co had a good year I invested - I will weep if I say how much, so let’s just say “a large sum” - in a property bridging loan fund a banking person said was safe and high performing. It did very well for several months until, suddenly, it didn’t and collapsed. I lost a lot of money, and learned that if the amounts are big I should invest only in things I understood, that I needed to understand risk better. These days I think about what would “minimise my future regret” in future investments.