Updated: April 3, 2019
Well, currently that would be Sogotrade. They have a lightning fast interface, inexpensive trade commission structure and a neat referral program.
They don't have a large marketing budget but they provide great service. The only reason I am writing this is because now and then people ask me this question and having used several brokers over the years (Scottrade, Acorns, Sogotrade, OptionsXpress, Zecco --> TradeKing and MB Trading (last 2 both now with Ally Invest)), I feel that the best and simplest happens to be Sogotrade, with a little bit of effort.
MB Trading used to be great for sophisticated investing but it is too much work to learn for regular long term buyers.
I am also in the process of evaluating Interactive Brokers, and once I evaluate them and also obtain a referring relationship, plan to add links here.
So the simple verdict is Sogotrade. However, if you want the comfort of a nearby office, then Sogotrade is good (now owned by Ameritrade)
