Potatoe mashers get put through some hard forces when used. They are forced down with pressure that can bend or compromise the masher in a very short period of time.
Try and stay away from the aluminum mashers they just are not strong enough and tend to bend. Plastic mashers are lighter and stronger. Like most plastics make sure it is BPA-free.
With that said stainless steel is the choice when it comes to the best materials in a potatoes masher. You want some weight with your masher so its easier to mash the potatoes. It is