I actually find that Kiefer Sutherland's voice is very fitting for Slade. Sutherland is an actor known for playing Jack Bauer in 24, and Punished Snake in Metal Gear Solid V: The Phantom Pain. He's got that older 'vet' type voice, the type that just speaks of experience, and by effect, speaks of genuine wisdom.

I like Ron Pearlman's performance in Teen Titans, but his voice doesn't really fit Slade's face if you ask me. That's the general problem with a lot of Deathstroke voices; they cast with the mask in mind, but when the mask comes off and you see Slade, it suddenly doesn't fit. They all seem too young for an old guy like Slade. But Sutherland is grizzled.

Sometimes though, when he is in costume, I can't help but slip into Ron Pearlman. That performance is just too iconic.

I think it works now, considering Slade's ongoing, to use Pearlman's voice. With that said, it's a timeless voice as well. Fitting for an older or young man (who happens to have a deep voice).

Probably Mark Rolston. Yeah, definitely him. I loved his perfomance in Arkham games as Deathstroke. It also both works, the voice and Slade's face. :)

I imagine the guy from Arkham Origins

Ron Perlman

Doug Cockle, the guy who voices Geralt of Rivia.

Scott Glenn

perlman