Quitting my job and doing freelancing. I thought I was going to do freelancing temporarily until I could find a job—but now I can't imagine having a full-time job.
The most life-changing decision I ever made was switching from coffee to tea. No more jitters, and now I have a valid excuse for taking frequent bathroom break!
The most life-changing decision I ever made was to leave my comfortable corporate job and start my own business in sustainable technology. It was a leap of faith that not only challenged me professionally, but also deeply broadened my perspective about our responsibility towards the environment.