What's the Best Piece of Advice You've Ever Taken?
1. It's advice that you followed, and it had a significant positive impact on your life.
2. It's not something you would have figured out on your own in time to implement it.
Also, please stick to advice that is at least somewhat general rather than entirely specific to you. If someone told you, "Hey, you should go out with [the person who would later become your spouse]," that might have been good advice, but it's not very interesting for other people to read.
For me, the best advice was to take an Accounting class in college. I don't even remember who told me to do this. But when I was putting together my slack-off schedule for the second semester of my senior year, I dimly recalled someone suggesting it and figured it might be useful. Ten years later, being the top Econ student at Georgetown and getting straight As in every Econ class as well as every class in my Computer Science minor has earned me precisely $0. But I've made a lucrative career out of the one Introduction to Accounting class I took on a lark because of that advice someone gave me.