What is the proper order of layers for bathroom walls?
Confirming we are talking about your bathroom, not the shower/tub area.

In honesty it doesn't matter.  Purple, green, regular drywall will all eventually act the same given moisture.  Does it taking more time really help?  Most building codes require purple/green drywall.  So put that up to pass inspection.  If you want to not have to worry about mold issues then you need to create a barrier.  

Two heavy handed oil based primer coats are the most effective way to do this (Killz for example - tons of brands that can do this).  This is why you see 100 year old bathrooms in great shape.  The oil based paint creates a moisture barrier.  Paint whatever you want over top of it.  Depending on size of bathroom (smaller ones I do this for sure) I will either go oil based everywhere and for sure on all ceilings.  

So to answer your question the steps are - insulation, drywall, oil primer, paint.  You don't put a plastic barrier behind drywall unless you are worried about moisture from the other side.