9 ball.
The secret to this game is Whitey. 9ball teaches HAVING to play shape on the next ball no matter WHERE it is. You have no option. Imho, no other game ( discounting the other rotation games ) teaches this massively important mind-set.
Others will pounce on this. Regardless, those pounces will be their opinion. This one is mine.
10 Ball; because it's called shots, and one more ball to run than 9 Ball. I'd rather play 9 Ball, or 10 Ball than 8 Ball. But then again, I like 8 Ball too. I find 8 Ball to be easier for me, but 9 Ball to be better practice for me.