Vodka
The tofu of spirits, and that's the point.
Why neutrality is useful
Vodka's lack of assertive flavor lets it sit beside almost anything, which is exactly why it spans drinks as different as the Martini, the Bloody Mary, and the White Russian. It also complements other spirits rather than fighting them, which is not something you can say about most bottles on the shelf.
What it's made from
Anything fermentable: rye, barley, wheat, beets, potatoes. Distillers generally use whatever's local and available. Potato's high starch gives a creamy, almost oily mouthfeel and a faint natural sweetness. Grain tends to produce a cleaner spirit that's easier to work with.
The opportunity
That ease is precisely what makes parts of the craft world sneer at vodka and refuse to touch it. Which, predictably, left an opening, and the mixologists who walked through it have built genuinely interesting drinks with unusual ingredients that convert skeptics.