i personally think the choice of fat/oil matters less than the method - which is critical for good roasties. use the right potatoes - floury like king edwards or marfona - cut in big chunks, par boil, then shake in pan so they go all bitty. heat oil/fat of choice in oven tray to high temp throw in spuds and cook on high for at least 45 mins so they go all crispy. i use groundnut oil but sometimes chicken fat if i'm roasting a chicken at the same time. some swear by goose fat etc - it's all a matter of personal taste