Back when I had money the best decision I made was to buy gold bullion, which I held onto for several years and watched appreciate. Had I thought things through, I could still have it, but I sold it to live several years later.
The worst single financial decision I made was in the commodities markets, as a trader using large amounts of margin. I had educated myself on market analysis and had watched one particular market with interest. The technical data screamed that market was going to fall soon, and probably sharply. When a key report came out, I was up, in the wee hours Aussie time to see what happened.
Indeed it did fall - sharply. I short sold as many contracts as my account could afford. Within a few hours, I was 30K richer. I was right...and this was going to be a windfall for me. I also was drinking at the time. Lack of judgement predominated and I sold even more contracts to maximize the profit from a steep decline. Unfortunately, the market made a rather large short term correction upwards and my position being too high for my account, I was squeezed out. Later that day the market fell even further below the rally line. I could have made 50-60K in that one trade but instead my account was toast.