Straight pool, for that is the game I hold in the highest regard and I think the skills required to be the best at it carry over to other games the most.
Unlike some, I'm wasn't looking at this question from the perspective of making money, otherwise the easy answer is 9/10 ball. I just looked at it from the perspective of which game I'd like to be the best in the world at.
Let's face it, if you were the best at any one of these games you could probably do pretty well financially in any of the others, whether in tournament play or straight action.