Game replayability.

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If there's stuff to unlock then it totally keeps be coming back. Ouendan is a really good example because of the bonus pictures, ranks, and levels that you can unlock as well as improving your score. Sadly, once you've Sed ranked everything though, it sort of loses a reason to make you come back to it.

Heck, in general music games are good for re-playability since it's usually just individual levels you can go back and replay. I also had a ball with Gitaroo Man and Space Channel 5.

Since the games are so linear it takes awhile, but I do go back to the Phoenix games occasionally to experience the story over again.

Oh, Pokemon is of course one because I try to mix up my team a bit and see how it goes.

Since I love the game more than life itself I love playing the first PS2 Devil Summoner. That game matches me perfectly in terms of what I enjoy; a good story, humor, cool character designs, multiple demons to recruit, and a sexy mad scientist.