My likes in order would be straight, 9 ball, one pocket, and last but not least, 8 ball.
Straight pool and 8 ball give you a number of balls to choose from when playing the game. 9 ball is more challenging in a sense because it is a rotation type game and depends heavily on controlling "whitey".
One pocket is a very interesting game and is fast becoming the game I enjoy playing the most. It's really makes you think prior to making a move. You may be able to pocket a ball but if you don't leave yourself another shot, what good is it? You may want to leave the ball hanging and leave your opponent safe instead. You really have to be careful in not leaving your opponent a bank shot and look to make sure there aren't any dead balls in the rack, As I said, you really have to think prior to making any kind of a shot. The only negative part of the game is when you start going down table. It can really lengthen the time of the game and sometimes get a little boring, but that's the nature of the game, waiting for your opponent to make a mistake that you can capitalize on.