What type of wood flooring material should go down on joists in a bathroom?

[kettlejim]

Is it best to use ordinary 18mm chipboard to go straight onto the joists or would it be better to use the OSB subflooring material or plywood and should any of them be waterproof or is this just the shower panels that need the waterproofing/water resistance?

I believe it's possible to get waterproof chipboard as well although not sure about OSB

 

[motorbiking]

Lay treated floor boards and then use a foam filled tile backer board on top.

 

[johnny2007]

I successfully used 18mm hardwood plywood on many occasions.
Treated and screwed down properly will never move.
Tested for 20 years on my jobs, no cracks in grout, no movement, no sqeak.