It's the World 9-ball Championship., the only one of these in which nearly all the elite players of both Europe and Asia enter, typically joined by the handful of American players who enjoy high world rankings.
The US Open 9-ball is closing the gap, and may be the answer to this question within a few years. Matchroom is building something very sepcial here, but at least for now, there's quite a bit of dead money in the field.
For now, the China Open is, more or less, on a par with the US Open 9-ball, with a smaller, yet very elite field.
The International is slowly evolving into a killer event, but isn't there yet, There's plenty of dead money in the field of 128.
So, for now:
1. World 9-ball Championship
2A US Open 9-ball
2B China Open 9-ball
4 International 9-Ball
Yes, the World 9-ball Championship is the holy grail in 9-ball, but I think that could change.