I didn't expect to like Ken as much as I did. There was sort of an "I've stood where you're standing and I know how hard it is" vibe about him that I really liked.
Mike was fine, though he seemed more disappointed in wrong responses than Ken did--sort of "well, everybody knows that..."
Katie has great rapport with the contestants in their interviews; I'd be surprised if she hadn't. I like her cheerfulness, which seems to put the contestants at ease (my wife said: "She's having a great time, and because of that, so are they!"). But I don't get any sense of authority. I know perfectly well that *all* the hosts take cues from the production table, but it sounds like she wouldn't know if a response was correct or not without the cues.
If I had to choose among the three, it'd be Ken without question.