Depends what you mean by best. The biggest ipod you can get is the 60gb version, and there is a 30gb as well, but the nano is the smallest "proper" ipod, and comes in 1,2,and 4gb sizes.
The Nanos are flash based with less storage but very small. The 5G (video) iPods have hard drives and thus much more storage but considerably bigger. Depends what you want I guess.
iPod 30GB - Best for a reasonable size collection of Music, Videos and Photos
iPod 60GB - Best for a large collection of Music or a large mix of all three
iPod shuffle - a good starter device for younger people or those who don't need complex features
iPod nano - a good compromise between the iPod shuffle and the normal iPod. Big features, smaller flash based size.