The best kind of potatoes to use are Russet potatoes. They are nice and starchy and work great in mashed potatoes. The next best option is Yukon gold potatoes. They are a little less starchy than Russets and won't be quite as creamy. Another option that works GREAT is to use a mix of Russet potatoes and Yukon gold in a 50/50 split. If you happen to have both types of potatoes on hand then this is a great option as you get the starchiness of the Russets with the buttery flavor of the Yukon golds.