I am surprised that 9-ball got so few votes. For me it was a toss-up between 9-ball and one-pocket. In nine ball you have to be able to come with shots all the time and you have to have every skill possible and be able to execute.
In rotation a foul does not give up ball in hand anywhere on the table and the scoring sytem is different. You do not have to run out 15 balls to win, you only need to get to 61 points first. In nine ball there is only 1 ball that wins the game. Nine ball is also the luckiest game which means that it is the hardest to weather the rolls. When a shot does finally present itself the execution must be flawless.
It is no accident that the best nine ball players also excel at one pocket and straight pool while the reverse is not always the case.
But - I chose one pocket because I fee as though it really incorporates all of the skills needed to play any game. To be successful in one pocket the player MUST be able to shoot, he must be able to hold the cueball, he must be able to masse' and carom and understand combinations, he must be able to read the stack correctly. One pocket is such a beautiful combination of skill and craftiness that it gets my vote for most difficult game out of those in the poll.
One thing that I never see mentioned is that one pocket may also be the most strenuous game. I say this because in one pocket there is constant shooting. A single one pocket game may have dozens or hundreds of shots - back and forth. In all the other games there is the possibility of long runs or a majority of runouts by one of the players even when the players are evenly matched up. This is rarely the case in one pocket. I believe that a session of one pocket may very well be more exhausting physically and mentally than any other game.
John