A masher is perfectly adequate for making mashed potatoes, and a lot depends on your technique. Make sure you get into the corners of the pot using a press-and-twist motion, adding a little liquid if necessary.
Your potatoes should also be properly cooked to get the best result.
The verdict: If you like your mash with a little texture, and like adding herbs or cheese, a masher is your best bet, but if you want it fluffy and smooth, a ricer is the tool for you.
A ricer forces the cooked potato through small holes resulting in rice-like pieces. Air is incorporated as it’s pressed through the holes, making the lightest, lump free, smooth mash possible. It can be a little time-consuming, though, especially if you’re using unpeeled potatoes.
A ricer is good for making gnocchi, as you don’t want leaden lumps of dough.