Potatoes!
Following that in a tie are carrots, onions, brussels (but oddy not cabbage unless in a slaw.), corn, & tomatoes (I’m not keeping the fruit ones separate)
Probably should differentiate 'tween cooked & salad:
Fav salad stuff
Iceberg Lettuce! then Romaine, Bibb, & spinach (never Kale!)
Carrots, Onion, Radish, Cukes, tomato, leeks, scallions.
I like carrots alright, but prefer them raw. The only time I eat cooked carrots is in chicken noodle soup… and the only time I eat chicken noodle soup is when I am sick 
Yeah, raw or pickled here too, or the other extreme - fondant.
Never had those. Looks interesting (minus the caraway).
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 to z!
Good point about if we are talking about culinary vegetables or botany vegetables. Botany vegetables would exclude the likes of tomatoes and mushrooms.
Noted, it was said “You’re allowed spices and herbs, but no accent veggies (like shallots or garlic).”
Hmmm… why are you all your favorite vegetables can “roll” around? 
I was just thinking a single choice amongst ALL veggies. By opening up to tubers, brassica, legumes, etc., you then have multiple choices.
This!
I mean, it’s a bit of fun, and also suspension of reality (re all the health-related comments).
Since peppers and tomatoes are fruits, I’ll go with Brussels sprouts. Or maybe fennel.
Corn (preferably on the cob) if I must include starchy vegetables, otherwise it would be okra.
Exactly. I always assume the latter with the types of questions that start with “if you had to choose only one…”
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!
For a very strict and narrow answer, shallots.
Fro vegetables that are commonly used as a side, broccoli or artichokes.
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.
Okay. I’m going to go with sugar snap peas.
