Board Thread:Questions and Answers/@comment-148.109.1.11-20131219190545/@comment-24438827-20131224044215

Super easy: Yuki, Miku, Rin, Yukari, YOHIOloid, Len Power

Kinda easy: Luka, Piko, IA

Hardish: Len, Gakupo, Kaito Straight, Rin Power

REALLY HARD: VY2, Meiko(mostly because of the V1 interface), SeeU

Note: these are just my experiences from trials and playing with my friend's vocaloids.

Of course, depending on what kind of music you want to make there are different vocaloids that would fit best. For instance, Miku would fit most genres, whereas Yuki might not.