The potatoes are low-fat, but the cream's not
I think Yukon Golds are perfect for this dish. Actually, I think Yukon Golds are perfect for a lot of dishes. They have a sweet, nutty flavor, and they're a great balance between being starchy —so they get soft and creamy when cooked—and being waxy (like a boiling potato)—so they keep their shape better than baking potatoes. But if you can't find Yukons, do use a starchier potato, which should be labeled baking potato, Idaho, russet, or russet Burbank. You want the potato to have some starch to mingle with the cream during cooking, and you want the slices themselves to have a buttery-soft final texture.
As for the cream, let me say this: if you're looking for a low-fat dish, this is not it. This dish is full of cream, and no, there's no substitute. There are versions of potato gratin that use half-and-half, or even stock, but for the kind of dish that makes your guests keep saying, "Oh, I think I'll just have one more little slice," and makes you keep thinking, "Oh, I hope they don't eat the whole thing—I want some for breakfast," you need real cream, and lots of it. And there's a technical reason for using cream, too: when a milk product is cooked with an acid (potatoes are quite acid), it can curdle unless the butterfat content is 25% or higher.
I put a delicate layer of shredded Gruyère cheese on top of the dish. Notice I say delicate, because if the cheese is too coarsely shredded or if the layer of it is too thick, it can get tough and separate from the underlying potatoes. So use the fine holes on your grater and don't be tempted to load up on the cheese. Imported Gruyère, Emmental, or Comté will have the perfect nutty-mellow flavor.