Hardly anyone eats Choko any more, more's the pity! Americans might know it as Chayote. It's a fairly bland vegetable, not unlike a patty-pan squash in flavour and texture, but it's far more versatile when used as a substitute for other things, like apples, or pears! It goes wonderfully well in pickles, too. I like it boiled, then served with lashings of butter, and a sprinkle of pepper, but it's nice with a white sauce, too.
Parsnips are definitely under-rated, too. I love them baked, but they can also be used in sweet dishes.
Arrowroot. Not many people have tried this one, but it's not unlike potato, except that it takes on the flavour of anything it's cooked with. A good padder-outer, but nice baked like a potato.
Squash. NOT pumpkin, but pattypan squash, marrows, spaghetti squash etc. These days you only see the pattypans, but there are much larger varieties. In my long-past childhood, we often had these stuffed and baked.
Hardly ever mentioned in cooking forums are the Chinese veges - bok choy, pak choy, tatsoi, choy sum, Chinese broccoli, kangkong and many more. Here in Australia they are widely popular and readily available, and I grow some of them for use in stews, stirfries, or raw in salads etc.
Beetroot - doesn't anybody else use them as a baked vege, or just boiled as an ordinary vege? Or does everybody have to have them pickled? They are SO nice when served with a white sauce.
Pumpkin is one vegetable that ought to be banned from the planet. Turnip is another.
