It really important to choose the right variety of potato. You want a floury potato, Maris Pipers or King Edwards are my favourite, but you can also use a Rooster, they all roast really well. A Russet, Yukon Gold or Red Potato are great alternatives if you are in the USA. I don't recommend using a waxy potato, while these are fine for baking, they simply don't fluff up in quite the same way as a floury potato like a King Edward or Maris Piper does.