The character Deathstroke or Slade Wilson is a better character than Black Canary due to his story line and his ability to defeat team arrow in minutes.
Deathstroke has had a much harder life than Black Canary. He was originally good but the combination of an experimental drugs and being lied to about his love lead him to a darker path. He can beat Black Canary easily in a fight with team Arrow. Alone she is no match for him. He is out for vengeance and this is the only reason Black Canary has survived.
Black Canary or Sarah Lance is a better character than Deathstroke. Deathstroke is a villain who has killed many people out of revenge which is not justifiable because he kills people who had no correlation to Shado's death. Black Canary had a hard life too. She was forced to live on a boat for a year with a crazy doctor that was the only protection she had from being killed or worse. I admit that Deathstroke had a hard life, but that does not excuse his actions. Just because he can beat up team Arrow does not mean that he is better than them.
Deathstroke kills people that stand in his way of Oliver Queen and his revenge against them. Black Canary was part of the League of Assassins who killed people for no absolute reason. He has an excuse unlike Black Canary. Black Canary helped Ivo torture prisoners. It's Sarah's fault for turning Deathstroke into what he is now. He is better at fighting which is why he can beat up team arrow which is why he is better.
Black Canary stepped away from the League of Assassins and Deathstroke is still killing people and sending other people to kill innocents. She might be back with the League, but she was no longer happy with killing people for simple reasons. It was not Sarah's fault that Ivo decided to kill Shado. She was in the wrong place at the wrong time. Deathstroke is the only one in control of what he does and due to the drug he took, he has degenerated into a crazy killing machine bent on hurting Olver Queen and killing everyone he loves. Deathstroke does not have good motives and even though he can fight better that does not make his character better.