Tony Robbins went from working as a janitor to being a multi-millionaire who sells inspiration for a living. And Warren Buffett, one of the most successful investors of all time, is currently worth almost $75 billion. Both of these legendary businessmen say the best investment they ever made was not a stock or financial asset. It was a class. When Robbins was 17 and earning $40 a week, he spent $35 for a three-hour seminar with personal development coach Jim Rohn.

Tony Robbins Photo by Santiago Felipe

The course taught Robbins that the most important work he could do in his life was on himself. "[Jim Rohn] made me stop focusing on what was outside of my control … and taught me to focus instead on what I could control," Robbins writes in his book "Money: Master the Game." "I could improve myself; I could find a way to serve, a way to do more, a way to become better, a way to add value." Buffett learned a similar lesson. He was terrified of public speaking when he was young. To force himself to face his fears, at age 20, Buffett signed up for a public speaking course with the Dale Carnegie institute.