What is the best online brokerage for US stock market dollar cost averaging of index fund?

Unfortunately there isn't a way to dca automatically into US index ETFs, if that is what you are looking at. You will need to wire the funds in and manually purchase the chosen counter on a periodic basis. Online brokerages wise, you can look at Td Ameritrade or interactive brokers. I'm personally using TD more for options portfolio management though and it's great as there's no live data fees or custodian fees