There are some truly retarded answers here, let me give you the straight dope.
There are three types of brokers.
Bucket shop brokers - Are effectively trading against you, they call these “dealing desk” brokers, or try and hide what they are, but effectively these guys know that 90% of traders lose, so they get all your money by trading against you instead of a small fee. These are the brokers offering you bonuses for singing up, advertising on the internet, with many fake reviews. It’s a trap! If you manage to win, these guys will stall and stall about paying you out, they are mostly regulated in countries without much regulation.
Real ECN Brokers - The interbank FX market trades in 1 million dollar lots. These brokers perform a service for a fee, they buy and sell million dollar lots, and break it down into the small chunks retail traders need. Fees are low, but they make money whether you win or lose, so they have no dog in your fight. This is what the retail broker should be looking for.
Prime Brokers - This is what the hedge funds and banks and large private funds use. You need at least $5m and probably $20m in the real world to be using them. This is above my pay grade.
Few brokers I know is a legit ECN broker, they are financially stable, and generally have the best practice for not f*cking clients. You can get them here https://finowiz.com/ . A significant proportion of what I would regard as the “serious retail traders” use them.