These are always served alongside something else, usually beef of some sort. They're great with steak, and sublime with pot roast. Of course, this isn't an exact recipe, and is quite flexible.
1 large potato per person
some butter (1/2 T per person; more will be tastier, less will be healthier)
1/4 c. milk per person (I don't measure anymore-- just splash some in and if it needs more, add more.)
a dollop of sour cream
Salt and pepper to taste
1/2 T. horseradish per person
Note: Horseradish, not horseradish sauce. You want something that will clear out your sinuses, not something that's mostly mayonnaise.
Scrub and boil potatoes until they feel done when you put a fork into them; drain. Add all other ingredients (less at first), and mash or beat with an electric mixer. I like them with skins on; you can peel them if you prefer.
If you slice the potatoes into quarters, they should boil in about 20 minutes. Larger chunks will be done slower; smaller, faster.