I don't really regret any of my purchases/investments. Sure, I lost money on most, gained some on others, and developed brands out of the rest, but each one, even the complete losses, were educational.
What I know now is because of those mistakes.
I suppose if I had to pick 1 though,
it would be choosing the .ws over the .com for my very first investment in 2005. It took 6 to 12 months for me to grab the .com and push the brand forward.
I still don't regret it though. It was an interesting learning experience back then.
The industry has changed a lot since then though. We didn't have the new gtld craze back then.
I would imagine, way more mistakes are made now days, with more options, than when I first started.
Heck, I still make mistakes in a constantly evolving industry, 14 years later.