What is the world's best investment? I'll give you a hint: it's not real estate. It's not stocks. It's not your brother-in-law's startup business.

It's you.

Yes, you can get financially rich investing in real estate. You can make a killing investing in the right stock or building the right business. You can save up money and invest it into mutual funds and retire wealthy. But none of that matters if you don't invest in yourself.

Related: How to Get Un-Stuck: 10 Key Questions for Self-Reflection

So today I wanted to break down just what it means to invest in oneself, and share four specific ways you can do so.

1. Your spouse

Perhaps more than any other single force on earth, your spouse will be integral in defining your future. But how much do we really invest in our spouses? Of course, I'm not talking about investing your cash with your spouse, (though, sometimes that might be a wise choice) I'm talking about investing your time, emotions and personal space.

Have you sacrificed what you want and invested it into what he or she wants? Have you given proper weight to his or her goals in life, not just your own? Have you taken enough vacations? Told your spouse you loved him or her enough?

Investing in your spouse matters if you want to live a long, successful, happy life.

2. Your friends

As Jim Rohn famously said, "You are the average of the five people you spend the most time with." So who are you spending your time with? Are you investing time with those who can make you a better person or with those who only want to keep you at their level?

Related: 3 Investments Entrepreneurs Need to Make in Themselves

3. Your library

I don't believe I've ever read a book and found myself unable to pull out some lesson to improve my life. That applies to both fiction and non-fiction books.

For this reason, reading is one of the greatest investments you can make, yet so many entrepreneurs complain they are "too busy to read." Don't fall for that falacy. Invest money in books and invest the time needed to read them.

4. Your health

Your spouse, your friends and your library won't mean anything if you are dead. That's why your own health is a non-negotiable investment you must make daily -- or suffer the consequences.

You don't need to run a marathon to begin working on your health. Small changes, compounded daily, will add years to your life. Begin by simply making an effort to move more frequently. Buy yourself a pedometer and aim for 10,000 steps a day.

Stop eating so many carbs, so much sugar and so few vegetables. Invest some time into learning how to improve your diet, and invest some money into buying the right kinds of food to keep in your pantry.

At the end of your life, you'll never regret not investing in that one stock, not investing in that one duplex or not investing enough time at work. However, you will regret not investing in your spouse, your friends, your library and your health.

The best part is that if you invest properly in these four areas, the rest of your goals (and investments) will likely fall into place. So make an effort today to start investing in the things that really matter and you'll experience the benefits for the rest of your life.

Related: Work Hard, But Don't Lose Your Sense of Fulfillment and Empowerment