Starchy or all-purpose potatoes are the best potatoes for mashing, although starchy potatoes break down better for creamier mashed potatoes. Russet is a good starchy potato, and Yukon Gold is a good all-purpose potato. Waxy potatoes do not mash well.
To make good mashed potatoes, start with starchy or all-purpose potatoes, and cut them into equal chunks. Chunks of irregular size won't cook at the same rate, so some may be undercooked and won't mash well. A ricer or masher works better than a food processor for making creamy potatoes, and adding salt, milk and butter at room temperature makes tasty mashed potatoes.