I'm a bit surprised, that the score for Mark Sheppard in here is so very low, because IMHO he's very much of scene stealer who always makes a great impression.
I've seen Mark Sheppard in "Supernatural", "Leverage","Chuck", ", "Firefly", "Burn Notice", "White Collar" and "The Middleman" (wow, I hadn't realized in how many great and exciting shows he's been in) and I've enjoyed every single second he was on the screen. He's got this incredible presence. He's got something very charming about him which makes all of his roles very convincengly evil and his Crowley for instance really creepy. That actually makes him the perfect bad guy. I could imagine him playing nice roles as well though (and I guess, he must have done that, I've only just seen him playing villains so far), since despite all the sinisterness he's impersionating, he's got something likeable about him as well. His Jim Sterling in "Leverage" for example was a role where he wasn't just vicious. Slimy maybe, not really honest, but the scene I loved him most in was, when he realized that Nate had been shot in the last episode of season 2 and tried to protect him. He really was believeable in this scene as well, because he never really would let you hate Sterling too much in the first place.
One last thing. I'm totally into genuine English accents and I'm very much of a voice person and Mark Sheppard provides both a great dialect and a great voice.