I never saw Steve Davis in his heyday, and only really saw the latter end of Stephen Hendry's career, so I'm probably not the best person to assess. In terms of longevity, Steve Davis was competitive from 1980 until as late as 2005 when he got to the UK championships, so that has to count for something.
For sheer dominance, Stephen Hendry. For flair, ability and natural talent, Ronnie O'Sullivan. So, for myself, I'd probably place the order as Hendry, O'Sullivan, Davis -- but, should Ronnie pick up a couple more World titles and other majors then he'll surely overtake Hendry, if he hasn't already. In the last couple of years Ronnie has shown some of his best snooker -- almost a more mature and complete player now than he was before. He could go on to dominate the big competitions for another few years yet. I do hope so, he's well worth watching.