If you want the most space, the iPod Classic is the best one to get. However, if you are looking for one that does more than just play music, you may want to go for the iPod Touch. It really just depends on your needs, since they are all good for what they're meant to do.
iPod touch, Don't go with an iPad it is horrible just a waste of money, it's the exact same thing as an iPod just in a bigger version. iPod touch is small compact that can fit in your pocket, iPad you'll have to carry around with you in your hands or backpack.