What is the most suitable material to use for baseboards in bathroom
Real wood.  Poplar trim is what I use for bathrooms or basements.  It is hard and holds up to moisture well.  If you get the plastic stuff it will look like... plastic.  It would have to really be in modern room or business setting to look right.

I would suggest getting poplar (pine is cheap and has moisture bulging issues) or some other hard wood.  Give it a first coat with drylok.   Follow up with whatever you want.  I would suggest an oil based paint for bathroom trim though - it is tougher and cleans up better.