Balls of any kind are generally easy to work with because they are the same no matter which side you approach it from and there have been so many ball type robot games in the past that people know what to do with them (in fact FTC used wiffle balls for the last two years I believe, and VEX used foam balls this past year). That being said every ball game I've ever played has been fun because there are usually many ways to approach it.
Never used hockey pucks as a game piece before, and it would be entirely dependent on what you would need to do with them. I think shooting them into goals like a giant air hockey match would be pretty cool. Keep in mind that an NHL hockey puck probably won't slide well on carpet or foam tiles.
I've had PVC batons as a game piece for savage soccer (a vex robot competition held by WPI) and they were a pretty fun game piece to play with. You could collect them off the floor or out of some dispensers, and you had to score them onto ledges up in the air. You can see the game here:
https://m.youtube.com/watch?v=swi6DPQl0qs
Any of the game pieces you listed could make for a great game, it's really about what you have teams do with them.
Edit: to actually answer your question, balls are probably the easiest for both a game design and a team getting into robotics. If you're looking for more challenging games, I would then look into the other two.