Just out of curiosity, in what order do you play the levels, and why?
Also, since it seems to be tradition for this kind of topics, here's my own answer:
Nifflas Forest, Gryphon Forest, Music Madness, Zepsi Industries.
I do this because Zepsi Industries is like a final boss's giant castle, and Nifflas Forest is like the protagonist's home. And also because of the challenges getting harder and stuff. That is also the reason i played this game (http://en.wikipedia.org/wiki/Castlevania_II:_Belmont%27s_Revenge) in the order crystal, sky, earth, plant.
Edit: And also because...
The Zepsi boss has different gameplay from the others.
I don't remember which pipe I started on, but after that I just didn't press any controls on the level selection screen and the ball bounced into all of them one by one! :)
Edit: Music Madness > Nifflas Forest > Gryphon Forest > Zepsi Industries > Nifflas Forest