Do these characters even have morals? If so, morals off. Stalemate setting. All fighting to their greatest abilities.
Well, writing wise, Deadpool isn't totally out of it. In the comic's he's in, he tends to be rather laid back, since he can't actually die. However, in Deadpool Kills Marvel, he was apparently holding back because of his insanity, allowing his ability to break the 4th wall and effectively use Metaknowledge to beat, well, everyone.
However, that being said, in a straight up fight, I'll probably have to give it to Deathstroke, since Slade has proven capable of beating some of the harder hitters of the DC Universe.