Vanguard has a lot of low cost index funds. They work during what I call "extended bank hours" so you might not be able to get your question answered late at night or on a weekend.
Fidelity has less low cost funds but some are lower than Vanguard and they have all the basics for a traditional 3 fund portfolio. Stay away from their financial advisers as they typically want to move you to paid services and high cost funds. You likely can get bonus money from them for opening an account. 24/7 support online chat, and phone. They have offices if you want to walk in.
Schwab has the lowest cost funds and has been lowest in ETFs for a long time. I have not found them to push to high cost or pay for service. 24/7 support and physical locations. I like their website a lot. They also have bonuses to open an account.
TDAmeritrade: Don't leave them out if you want Vanguard but hate hour long hold on phone calls and banker hours. TDA sells a bunch of Vanguard ETFs and if you sign up for free ETFs, there's no extra cost over being at Vanguard. They also have stores and bonuses for opening an account. The website is extremely busy with the front page showing you every piece of info that exists in the world, but once you get used to it, it works well. They also have 24/7 support.
I have accounts at all 4 of these. If I were starting from scratch right now, I'd consider all but Vanguard. While they have a bazillion low cost index funds, I only need 3 and everyone has them including TDA where I currently hold VTI.