I find that just about everything I prepare involves a cutting board, a chef's knife, and not much else aside from a bowl for mixing things. But, the knife is pretty obvious. I get a lot of use out of the toaster oven - which, like
GadgetGuy's, also has a convection oven setting. But, that's not something I use to prepare food...more like a quick reheat or a drunken invention late at night (hence the name).
I use the mandolin quite a lot because I pickle things a lot, and those things pickle better when they're sliced uniformly. I also use a box grater, or a small hand grater, quite a lot, whether it's cheese, vegetables, or garlic. I find grating the garlic to be much faster than dicing with a knife.
Oh, and a digital thermometer. There aren't many dishes I make that don't involve this.
There are a lot of least used items, but the spiralizer is one that I never used for an actual recipe. I once used it just to test what it would do with a zucchini. Okay, now I know. Some things I barely use - like a sous vide machine, vacuum sealer, and stick blender - but I still have some use for them, at least.
And, as for a stand mixer, this comes out whenever I'm baking, with an occasional exception. I didn't use it for cornbread muffins I made 2 days ago (I hand mixed the ingredients). But, I always do for sweet cupcakes, cookies, and bread. While I could hand mix to combine the ingredients, why take the extra time when I can do it so perfectly with a little bit of electricity?