Spuds from the farm up the road, copious amounts of kerrymaid, mashed to a smooth consistency then a dash of milk before mixing vigorously with a spoon. Anything else is sacrilege (excluding scallions).
Not too particular about the potatoes, as long as they're not overly floury. I use a handheld electric mixer, and about a cup of mixed milk and butter, plus salt to taste. Heat up the milk and butter first, seems to make it even creamier.