Do you believe you can consistently pick stocks that beat the market long term? Do you believe you can can pick a fund manager who can consistently pick stocks that beat the market long term once you account for their fees? Do you believe you can pick a financial advisor who can pick a fund manager who can consistently pick stocks that beat the market long term once you account for both of their fees? If you answered yes to any of those questions, I bet you're wrong. If you answered no to all of those questions, good job, you should invest in an index fund that tracks the overall market and has the lowest fees possible. Solution: Vanguard (or similar offerings from Fidelity and others)
A little tongue in cheek, but that's the basic premise of Vanguard (and other low fee index funds).
Edit: Also, Vanguard has a unique company structure whereby the investors own the company that does the investing. This means that they are legally and morally required to do what is best for their investors (those that own their funds). By contrast companies like Fidelity and Schwab, which also offer low fee index funds, are owned by people who own their company by owning their stock on the open market (Schwab) or by being part of a particular very rich family (Fidelity) which means they are legally and morally obligated to do what is best for their owners (but not necessarily their customers who invest in their funds). Luckily Vanguard has been so successful that what is best for Fidelity's and Schwab's owners is to offer low fee market based index funds (sometimes even lower fee than Vanguard), but only in as much as some potential customers demand it. So, at the end of the day the investment options are pretty much the same but philosophically Vanguard is more aligned with your interests. FWIW, my wife and I have most of our money with fidelity because it's convenient based on my solo 401k and her employer plan being their.