This is important. You want a broker whose clients speak highly of it. The same way you wouldn’t buy a car if all the feedback was that the brakes don’t work.
It’s worth scanning various forums to check out the reputation of a broker beforehand. However, you need to be cautious with commercial website rankings, as the owners of these sites often charge to rank a broker, and thus the more the broker pays, the higher the ranking.
An absolute must (in our opinion) should be that the broker is regulated. You don’t have to use a regulated broker, but we, and most others in the industry would highly recommend it. It adds to a broker's reputation if it is regulated.