Actually watching him. He's real quality. He's as accurate as any other spinner I've seen, changes his pace with no change in action, and can rip it big. The last fact alone gives him an edge on Vettori and Bhajji.And you base that on what?
Note though, by saying he is a better bowler I'm not implying he has a better temperament. Just better skillz. He could just as well end up bowling utter rubbish when the team needs him the most. I've seen him mostly in domestic cricket, old India matches and the IPL. He didn't seem to choke or appear toothless at International level whenever he played, and certainly deserved more chances than he got. It just seems the selectors thought he was too old by the time Jumbo retired and so went for Ojha instead.