Helmet - Absolute must , hard piste concusion isn't good & Powder can hide rocks below the surface!!
Impact shorts - Tried a few times riding without Impact shorts and just cant, protect your arse at all costs LOL
Knee Guards - If conditions are deep and powdery you can get away without these imo, if the pistes are hard i will always wear them.
Wrist guards - Rode with and without them dependent on conditions when learning and once confidence set in gave up on them entirely. Its a psychological thing and if you feel happier wearing them then that will help you learn quicker at the beginning imo. As others have mentioned thou the main thing is to try and roll with the fall and keep the shock out of extended wrist/ arms.
Elbow guards - Never wore them
Back protector - wore one when i started playing with kickers/ rails in the beginning. Confidence lets you move away from it, eventually
Have fun