What is the most important healthiest veggie you should be eating?

there is no 1 healthiest veggie. The person who suggested spinach apparently does not know that spinach contains oxilates which inhibits all that lovely iron from absorbing.

BUT if you eat the spinach with, say, tomatoes, the vitamin C in the tomatoes aids in iron absorption.
Thats the way it is with all veggies and fruits too. That is why it is recommended to eat a wide variety of fruits and veggies. They all have something different to offer the well balanced diet.

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eat a full range of fresh vegetable any that you cook should be steamed or oven cooked rather than boiled to preserve the vitamin content of the vegetable as most vitamins are water soluble.

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Most people don't know this but an artichoke is the healthiest vegetable you can eat. One artichoke is 60 calories and contains over 4 grams of protein. It also increases your sex drive.

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If you want a healthy diet, the key is not to have one "healthy" veggie... the key is to eat a variety of veggies that are a variety of vibrant colours.

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Sweet potatoes (fresh) or brussel sprouts rank quite high nutritionally across the board but any fresh vegetable is extremely healthy even white potatoes.