For me:
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Broccoli
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Carrots
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Onions (even if they make me gassy as a mofo)
I love American Chinese food and most of the dishes feature these three veggies so I always have a banger of a meal when I order.
Honorable mention to green peppers too, a very versatile veg that goes well in anything.
Bell pepper (green to red they're always delicious)
Broccoli
Beans (I don't care what kind they're all great)
Honorable mention to the jalapeno, and other mild Chile peppers they make a dish explode in flavour.