I know certain people will look at my view dubiously because I'm an obvious 'biased Kane fan' (see the avatar if there's any doubt), but of course I'm picking Kane here. The original Kane especially proves that Kane is the better 'monster' in my view because of the mask and the build up and intrigue WWE managed to create.
As some wise people have said, Kane's mask quite literally masked his emotions, so that fans could never really tell if he was in pain or not and indeed that is scary. Whenever a wrestler gets hit with a chair for example, you expect him to grimace in pain but with the original Kane, he just kept coming and of course, he never displayed emotion, for the first few years anyway.
I often compare the original masked Kane to Michael Myers, a large masked individual who never speaks but no matter how much you do to him, he can never be killed and that's kind of like what Kane was in the early years. To me, the old Kane was the best monster I have ever seen in wrestling and in that context, Abyss stands no chance.
With that being said if you compare Abyss to the Kane we saw the last few years, I would say Abyss was the better monster but again it's kind of an unfair comparison considering TNA often emphasized Abyss's monsterous qualities while WWE virtually ignored Kane, so of course it was going to be difficult for Kane to compete in the best monster competition.
However, when Kane is on his game, I think he is able to pull of the monster persona quite perfectly (see his work the last few months for example) and that's why I give the edge to Kane. Also, Kane's promos are often very articulate which makes him seem quite intelligent, a bit like a smart serial killer, whereas Abyss just goes off on a bit of a meaningless ramble, but I suppose it's really down to personal taste.
Of course, that doesn't mean Abyss isn't a good monster character because he does an excellent job of portraying the psycho he's supposed to be and he's one of the better TNA performers in my opinion but yeah, Kane for me.