Cabbage, probably. From pickling applications to the huge range of flavors that can be drawn out through different hot cooking methods, to its use when raw as a base for salad/slaw, it has won my heart.
Still learning, but I want to ask do we get to pick a species, or can we pick from a family or genus? I’d go with legumes, but brassica is a very close second.
First choice would include all legumes; sugar snaps, green beans, pigeon peas, but I would also love brassicas, especially cabbage and cauliflower. And then alliums like shallots, but not necessarily in that order. Not sure it would be my choice "vegetable ", but maybe my first choice ingredient.
If my own life span was not a concern , i’d choose potatoes.
If fruits (tomatoes, eggplants, zucchinis, cucumbers, peppers…) and starches are excluded (as they should rightly be unless one wants to die of a combination of malnutrition and boredom), I’d pick the leek. It’s available all year long, can be cooked and accommodated in a number of way : boiled, steamed, fried, braised…, eaten cold or warm, even raw in a salad, served as a stand-alone veg or in combination with other ingredients or just used to add flavor.
Should fruits and starches be included, I’d have no other choice than to go with the starches, probably rice rather than potatoes as it can at least be turned into deserts and sweets to alleviate the boredom.
A fun question, but nearly impossible, like choosing a favorite child or pet!! But, to answer best, I will have to firmly put potatoes into the starch category, because I couldn’t live without them. Also, tomatoes, and eggplants are fruit, and mushrooms are fungus, so they go there. Corn is a grain, yay!! That leaves alliums which I could never give up, and my other choice would be asparagus, I think. But I love them all!
Given the diversity of vegetables, this is impossible to narrow down to one. It would be between asparagus, green peas, broccolini (kind of cheating since I love both gai lan and broccoli, so the hybrid gives me both), and sweet potatoes.
Potato needs its own respect because of how versatile it is.