Well, for me mashed
potatoes are different than smashed
For mashed potatoes, I prefer russets because they can absorb a greater amount of fat & water based liquids while still remaining fluffy compared to waxy reds, "chefs", or yukon golds. I simmer them in salted water, drain and dry them in the hot pan, and then run them through my manual food-mill (similar in effect to a ricer) which produces a smooth & skinless puree. I try to work the potatoes as little as possible to prevent the starches from getting all gooey. For classic mashed, I then fold in warm butter, cream, kosher salt, and freshly ground white pepper - finely
ground - I hate hard bits of peppercorn in my potatoes. I usually use my Mortar & Pestle for this.
Of course I sometimes custom tailor these to the other courses with a million different variations. Compound butters, herbs, roasted garlic, truffle shavings, infused oils, reduced stock glaces, clarified animal fats, and so on. But when I want the flavor of the potatoes to really shine through, I like just butter, cream, kosher salt, and finely ground white pepper. Having the proper amount of fat is the most important part, as it seems to carry the earthy flavor the best.
When it comes to smashed potatoes, I cut reds with skins on into medium dice. Next I simmer them in salted water, drain 'em & dry them in the hot pan, pour warmed butter and cream over them, and lightly "smash them" with a hand masher. Again, I try not to work them too much. Dicing them before hand ensures you have the proper sized pieces of skin without having to overwork them afterwords with abusive mashing trying to break up the large skins. Being pre-diced into chunks, it takes only a few squashes for the desired texture. Then I add my kosher salt and freshly ground white pepper and gently fold it with a spatula to incorporate everything.
Man do I love potatoes...