Davina McCall is the outstanding presenter of her generation on TV. I've worked with her and there is no one in this sort of area to match her grasp of a brief and her innate understanding of television/people/society etc. The proof of the pudding is in the salary, the channels which employ her and the slots she is given. She has been trusted with both C4's and ITV1's biggest and most sensitive projects.
Dermot's very popular in the biz and has a good attitude but after years on TV he still makes avoidable mistakes in every piece he does and is not sufficiently well informed or fast enough to initiate or explore anything without resort to the production team. He's a kids' presenter basically and executives have never trusted him with a major peak time show on network TV. He might get there in the end since he's so likeable.
Russell is a comedian, not a presenter. His role is different and I see his career being more in comedy than light entertainment. This is good for profile, then he'll move on in the field that is his forte.