I am Canadian so in terms of the online brokers, I can only speak to what we have in the Canadian market. I go with one called “Questrade.”
Despite the name, it’s a great platform to be an index investor because they do not charge commissions for buying ETFs only for selling them. Since we want to become buy and hold investors, that is a good incentive structure.
If it were me, I would look for one of the well known online brokers that offer a similar fee structure (Free to buy index ETFs)