I recommend a potato with a medium starch content, like Yukon Golds. You can use whatever type of potatoes you like or have on hand. A higher starch content potatoes like a Russet would be fine, but the amount of starch may make the gravy get too thick. Likewise, a red potato with a relatively lower starch content may not add enough thickening to the dish. I've found that Yukon Golds are just about right. That being said, if all I had was Russets or Reds, that's what I'd go with.