Impossible. Onions, peppers and mushrooms are all great and can be put in a huge variety of dishes, but then root veg are perfect for roasts and stews. Too broad a spectrum of stuff to pick one.
Onions are probably the most versatile and I would miss them the most though I think. Good for adding flavour to loads of things, great when caramelised... add to soups, stews, stocks, curries, risottos, stir-fries, paellas, casseroles, jambalayas, raw in a salad, pickled, fried on a hotdog, rings in batter... the list is endless.
You know what? Screw it. I changed my mind on the 'impossible' thing. Onions. I pick onions.