This recipe makes REAL mashed potatoes that are anything but watery and tasteless.
Potatoes. Yukon Gold or russets are the best choices for smooth, fluffy mashed potatoes. Yellow-fleshed Yukon Gold provide a buttery color. If the potatoes have tender peels, simply scrub them and either leave the peels on or remove them after boiling. If you intend to run the boiled potatoes through a 'ricer', then peel the potatoes before cooking. If the potatoes are small to medium, leave them whole. If rather large, cut into quarters.
Mash or rice? If you don't mind potatoes with a few lumps, just use a potato masher. For perfectly smooth, lump-free potatoes, run them through a potato ricer (photo shows the ricer, a device that squeezes the boiled potatoes through a perforated disk). Some folks use an electric mixer to beat the boiled potatoes; I think that's overkill and produces gummy potatoes, but do as you like.
Milk, cream, butter. Use WHOLE milk, half-and-half, or light cream; low-fat milk doesn't cut it. As for butter, UNSALTED butter is sweeter. Skip the margarine.