For me I'd have to say it's been reading close to an hour of non-fiction daily and making it a habit.
I've just learnt so much stuff from some of the best minds and its translated into a better mentality as well as a deeper understanding of the various facets of life. I read a lot about productivity, money, time management, happiness, goals, approaching life, lifestyle design etc.
With that knowledge continually coming in, a cascading effect on the way I operate, and better life choices have come. All by simply reading 45mins per day.
What was the one lifestyle change you made that had the greatest impact on you?