Great question. I might have to do one per decade - but maybe that would give away my age, but I think leaving an abusive relationship might have been the best all time decision. Maybe it was quitting a job as a server where I was told not to chat with the people of color. (That was a long long time ago in the South.)
Wow! I have made so many life changing decisions. I doubt that I can choose just one. I have made some very hard ones that I am proud of, but when it comes to "best" I would have to say,"when I finely made up my mind to make the military a career". That was for me. I'm sure there were ones for others that they would think more important.