Helf

Leg because prosthetics are more versatile for legs than arms

Arm, as long as it's my left (non main). I genuinely think you can learn to do almost anything with just one arm. I guess gaming would be fucked and certain activities might be impossible, but I could learn to give that up do other things. However I find my mobility, balance, and ability to travel or run even in difficult terrain is far more intrinsically valuable.

The drummer from Def Leppard’s only got one arm...