TL;DR - If it's for an IRA, Fidelity, Schwab, and Vanguard are all great. If it's for a taxable account, they're still all great, but there are some important differences.

As others have said, consider Vanguard as well. Honestly, all three of them are solid and for a pure individual investment account, I can't really say which is best because they're all great. Some things that might push you toward one over the others:

• if it's for a taxable brokerage account - Vanguard mutual funds can be converted to ETFs without a taxable event
• Schwab's debit card is great (no foreign transaction fees, no ATM withdrawal fees worldwide)
• Fidelity's debit card is great (largely same as Schwab, but has a 1% foreign transaction fee)
• Fidelity has a 2% cash back credit card and a rewards debit card (Bloom; no international ATM fee rebate and higher foreign transaction fee; can have both Bloom and Fidelity debit cards)

If it's just for an IRA, none of the above matters and it all comes down to fund selection, and both are very good. If you're going to buy ETFs, it matters even less.