Deathstroke has become more popular in recent years after fighting the Teen Titans, becoming a main villain in Batman: Arkham Origins, and joining Arrow, as an ally in the first season, then a villain in the second.
Deathstroke is a rigid, steel willed man. His right eye is covered, either with his mask or an eyepatch. A ruthless mercenary, he acts as major villain for the Teen Titans, Batman, and recently Green Arrow. So, let's get on with it. Note these aren't in any specific order.
Best known for his roles in Gods and Generals and Avatar, Stephen Lang would be an excellent choice. He's pulled off the tough villain before, and I can totally see him rocking an eyepatch. Just looking at him you know he can make Batman quiver. If not Deathstroke, I think he should paly Cable for Fox. Both are similar physically, and he could pull off either. Let's hope he's Deathstroke though!
Ron Perlman was the voice of Deathstroke in Teen Titans, so I guess he has an edge on other contenders. He definitely looks the part, plus he has popularity from Sons of Anarchy. Definitely more well known than Stephen Lang.
Manu Bennett is definitely one of the reasons Deathstroke is important today. He starred in Seasons 1 and 2 of Arrow, and is returning for an unknown episode count in Season 3. I don't really see Deathstroke appearing in the DCCU until after 2020 (except for potential cameos in the Batman solo movie or Suicide Squad, but I feel like we would've heard of these by now). And by that point, or at least after Green Lantern in 2020, we'll hopefully have Crisis on Infinite Earths, where we could see the DC TV Universe mix with the Cinematic one.