It's a bit of a how long is a piece of string question with out more information or looking at it really.
Think about what the finish is going to be or if it has to blend in with anything exsisting. Plasterboard will need some sort of frame to support it meaning that you will lose some space in the cupboard. On the plus side once skimmed it will decorate with pretty much anything and will be relatively fireproof.
MDF will be fine if space is tight. Dependant on the size a piece of 18mm+ will be self supporting and will take decoration. If the area is large or there may just be a possibility of dampness or condensation then think about ply. I used a sheet of ply to form the walls of a very cramped toilet in my house, the toilet itsself just having been "plonked" at the rear of the garage by the outside door.
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