Ok, no one has touched mashed potatoes. I'll add my 2 cents.

The thing that generally makes bad mashed potatoes is that people try to cut back on the fat or the salt. More rarely, people fuck up the potatoes, usually by boiling whole, large potatoes instead of using very small potatoes or cubes. Also, people forget how much salt that potatoes can handle and so they do not salt the water that the potatoes boil in. And remember, start the water with the potatoes in it and then bring them to the boil together.

Anyway, back to fat. It doesn't matter what fat is used, but that fat is the first building block of flavor (on top of the potato flavor). The second is salt; how much I salt my potatoes during mashing, pureeing, or ricing depends on the flavor of the fat and if the fat is salted already. So, if I use salted butter, less or no salt. Olive oil? A lot of salt. Chives or green onions work very well in potatoes, as do things like garlic (particularly grated garlic), herbs, a little horseradish and/or mustard.

The next round of fat after the butter or drippings or whatever, is milk. I always use whole milk, if I'm using milk. The last batch of mashed potatoes I made used a mixture of buttermilk and sour cream and man oh man, it was good. Another thing I like to do, is to steep things in the milk or whatever liquid component. Leeks work well, as do chives or other herbs and sometimes I'll add all the seasoning to the liquid to gently steep for a while. Of course, if you do this, it has to be ready for when the potatoes get mashed.