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I use medium red potatoes and the recipe from Cucina Simpatica by George Germon and Joanne Killeen from Al Forno restaurant in Providence, RI.
I boil the potatoes. I mash them with an old-fashioned ricer. I add lots of heavy cream and butter. These mashed potatoes are pretty rustic, downright homey. For a fancypants mashed potato recipe, just google Joel Robuchon's potato puree.

I use either Idahoes or Yukon Gold for my mashed potatoes. If I'm doing smashed potatoes I use red bliss. I peel them, cut them up, and throw them into salted, boiling water.(I don't peel the red ones) I use a potato ricer or an old fashioned potato masher to mash them. I have made countless combos, but the basic mashed for me has lots of butter, salt, pepper, and either milk, cream or sour cream. I've thrown in:
sauteed onions, mushrooms
mashed turnips
mustard
chives, tarragon, basil, or other fresh herbs
cheeses
bacon
jalepeno peppers
leeks,spinach and other veggies
buttermilk
etc.
I also make mashed sweet potatoes, which has another set fo additions.

Russet or Yukon Gold
Boiled
Old fashion zig-zag potato masher
Add heavy cream, cream cheese, sour cream, butter & nutmeg

Russett or Yukon gold. Peel, cut into quarters and slow boil in liquid to just cover (sometimes I use half water and half chicken broth) until tender. Drain, mash and hand-mix with milk or cream, real butter and, if it's on hand, a little sour cream. S&P and it's done. Pretty simple. I like a few small lumps -- makes the potatoes stand up to the gravy.

I always add warm milk, pepper, nutmeg, salt, butter, and occasionally the smallest pinch of sugar. Yummm.

@PhotoKirk : you sir deserve a medal.

yukon golds for no skin, reds for skin on. boil until very fork tender, into a big mixing bowl, coursely cut into manageable pieces, add butter, go at it with an electric mixer or old fashioned masher, add milk or sour cream. from there it all depends on what i have laying around, or if i want it to taste like something specific. i'm a big fan of the parmesan-chive-roasted garlic combo, also, the cheddar-chive-bacon combo, and more recently the fontina-garlic-mushroom combo.

mashed sweet potatoes need only sweet butter, anything else is guilding the lilly really...

I use Ruby Reds (a lovely pinky red skinned waxy potato) - some times I'll go for a Royal Blue (dark purple skinned waxy). Chop 'em up, boil them till their almost falling apart. I used to use a potato smasher, but since I moved it got lost, and I've had to use a fork. I found I wasn't getting the smoothness that I wanted out of my potatoes, so I've been taking a hand whip to them. Boy, does it give your arm a work out and get your potatoes whipped smooth and airy. I drain them, add lashings of butter. Mash, them get some milk and warm it so it doesn't cool the potatoes down. I add that and whip until creamy. If I feel like it, I'll add some lovely sharp romano cheese grated real fine, at the same time as the butter, so it all melts down into the potatoes. It makes for cheesey-licious potatoes :)

No matter what type (garlic, plain with gravy, or that russet-sweet combo I've been itching to try) I always heat up the liquid and butter before adding it to the potatoes. I also over-estimate the liquid, and add it until the potatoes just won't take any more! Very, very creamy.

-yukon gold, boiled
-a RICER is an absolute requirement if you want them fluffy and not gummy
-I put a LOT of butter, a touch of cream and salt but if I tell you how much, I may get arrested by the butter police!

easier is better. Any kind of potatoes. Pinch of baking soda for fluffy, whole milk or buttermilk, plugra or vermont cheese company butter, sea salt

Casual, just for me and friends - Red potatoes, skin on - mashed up w/lots of butter and warmed sour cream. A good brown gravy poured on top.

Formal, when I'm trying to impress - Half red, half Yukon gold potatoes, skinned, cooked, put through a food mill, mixed with lots of butter, cream, buttermilk and minced scallions. They're really good, though I actually prefer the Casual spuds, doused in gravy. Yum.

My Grandma's Mashed Potato:

Boil cubed russets until tender in just enough water to cover them. Don't drain (you may need to boil some of the water out). Add a stick of margarine (I prefer butter - don't tell her that), milk, and just enough salt and finely ground pepper to taste, but make sure you leave the seasoning for the eater. Whip them with your beater until smooth and creamy.
Heaven is served simply and easily.