I don't think it is cut and dry as all that. The equipment they are playing on nowadays allows for a more attacking game. Try going into the pack on slower cloth and then again on fast cloth you'll see the difference. I know of a very good player who still considers Joe Davis the greatest ever because he was managing a dominating standard on far more difficult equipment.
That said, overall the players are far better now than ever. I don't think you can dispute that at all. Ronnie O'Sullivan is dealing with far superior competion than Steve Davis or Stephen Hendry were during their hey day. If Ronnie ever manages 6 or 7 world titles I think they would be worth more than Davis or Hendry's. It's just not possible to dominate in the same way anymore when more than half of the top 16 has made over 100 centuries in competition.
I would most certainly place Steve Davis and Stephen Hendry ahead of Ronnie. Steve is a superior match player and both have an excellent temperment. You wont see either give up either mentally or literally. For that reason, they will get through more difficult situations and win where Ronnie wouldn't.
Statistics don't tell the whole story. You can compare Ronnie to John Higgins as they've played their careers against the same people, but you can't compare Ronnie to Steve or even Steve to Stephen. Again they all played their primes against different players, and in some cases different equipment.
But to be fair, Steve Davis still holds the record for most professional tournament wins. Also he may have fewer ranking wins than Hendry, but there were also fewer ranking events available to him through a portion of his career, in fact there are bunch of tournaments he won before they recieved ranking status. He may have on less world title than Hendry but he does have one more UK title.
I think Steve had the more all around game, but if I was forced to choose I'd give the nod to Stephen Hendry. My reasoning is that Steve Davis perfected the style of snooker that was being played at the time, but Stephen Hendry took it another step further and revolutionized the game. I remember reading that Steve Davis slowly suffocated his opponents, whereas Stephen Hendry blugeoned them to death.
If Ronnie were ever to get his head together and play with consistency then I would be willing to give the nod to him. From 2007-2009 I thought he was the best I ever saw, but then he hasn't been playing that well since. Comparatively any ways. You have to maintain that standard for more than a couple of years.