Make sure that your broker is regulated in whichever country you’re in.
Execution should be pretty much smooth. You don’t want to have a system hang lack or whatever.
Customer service should be top-notch.
Ease of withdrawal, you put in money with them, you trust them. The money should come back in one piece.
Record everything, compare the price feeds on different brokers. You can share on social media, bring up to your broker or even the last one is to bring it up to the authorities.