Now the title is a bit forward. I’m sure that you read that and thought to yourself “geez Ric, what is the best investment you can make? Let me guess it’s yourself or something corny”. and well investing in yourself is the most important investment the best investment is in fact a monetary one, so put your pitchforks down (at least for now).
For most people, they have neither the time nor constitution in order to deal with complex financial equations and they would rather catch a hard stiff kick in the nuts then go though a prospectus, in that case we need to take into account what the best use of our time is. As such the answer to the question proposed in the title is S&P 500 ETF’s. One of my favorites (and pretty much anyone else who knows what an index fund is) is $SPY. Or many a Vanguard Admiral fund, if you’re a snob.
These funds are great because they can compress a wide array of stocks into one (relatively) small package that contains all the diversification (in equities) that you will ever need. Also they can be purchased from any broker and if your are feeling ballsy, you can buy options on them too (I would recommend to NOT DO THAT!), anyways index funds are a nice easy way for the stock market beginner to dip there toes into the deep end, and lets be honest make money is a lot better than paper trading on Investopedia
About your money and Ricky Rich is a not a financial ad visor, What I recommend is strictly a suggestion and nothing more. I am not responsible if your investments go tits up, seriously, do you and make sure to DO YOUR OWN RESEARCH BEFORE ENTERING IN A POSITION I AM NOT A FINANCIAL ADVISORY PROFESSIONAL!