It might seem strange that he ranks so high when he has only appeared on two pages in all of DC continuity, but he represents everything that I hate about comics. He was placed into an ending issue of Red Robin with seemingly no thought. He is a ten year old with no powers who beat Red Robin and Black Bat with ease. Note that Black Bat beat Lady Shiva who can beat Batman. How exactly can a ten year old do this? He is, in my opinion, a poorly written Gary Stu.
I don't really dislike Deathstroke as much as I dislike a particular aspect of him. To be more specific I dislike the "90% of the brain" part. That has always been a personal pet peeve of mine. I also like really strong characters to not be super geniuses. He can be a tactical genius, but I don't like that the world's deadliest mercenary can build fully functioning robots (same goes for Superman).