Saw Jamie do a taste test on tv - and the conclusion was that butter gave the best flavour and crispness compared to duck fat and olive oil, but I guess it is all down to personal taste. He also used a potato masher to lightly press them down into the butter or whatever, halfway through as that increased the surface area of the potatoes onto the butter, making them crispier, if slightly flat!
Did that at Christmas and found that they tended to break up rather than flatten, but I suspect that I had maybe parboiled them a bit too long.