I put peeled, quartered Russets in cold water, bring them to a boil, and cook until just tender. While they're cooking I gently heat equal parts buttermilk and butter together (about a cup each for 5 lbs of potatoes). It's ok if the buttermilk curdles a little. Once the potatoes are drained I put them in my stand mixer with the paddle, and carefully mash them (don't go crazy or you'll get glue!). Add in the butter/buttermilk mix, salt, pepper, and whip until just combined.
I think the buttermilk gives a fantastic tang. If I'm in the mood, I'll throw some finely minced garlic onto the hot potatoes, or shredded old cheddar. Or both. Yes, the butter is excessive, but this is not an everyday dish around our house.