Your first $25,000 should be invested in an all-market (VTI) or S&P 500 index fund (VOO) in your 401K or IRA.

Your first $500,000 should be invested in a broad market, low-cost, diversified portfolio of large/small cap and some international index funds.

After you reach $500k, you need to invest the majority of your portfolio into individual stocks. 
At this scale, tax efficiency becomes increasingly important, and you need an actively managed portfolio which optimizes the tax allocation of investments (growth stocks into taxable, REITs & bonds into 401K, etc), minimizes trading frequency, and practices tax loss harvesting (sell losses first to minimize capital gains tax). You can move your funds to a robo-trader and consider getting a dedicated advisor.

When your portfolio nears $1.5 million, avoid the tendency towards an overly risk-averse portfolio due to substantial swings in net worth caused by market volatility. Develop additional income streams from revenue-generating investments such as rental real estate, small businesses, etc.

Around $5 million, personally managing income streams may be an inefficient use of your time. Consider diversifying into hedge funds that offer positive absolute returns and research private equity opportunities which are promising and interesting to you. If you are older and your net worth exceeds $5.5 million, consider forming a trust fund to safely pass your fortune to your family.