If I could grow only one vegetable, my choice would be kale because it's productive year-round and I never get tired of eating it.
If you like some vegetables more than others, then the best vegetables to grow are the ones you like to eat. I like most plants, but I vastly prefer eating kale rather than lettuce or chard. In my gardens, I pick a few leaves here and there, and then the lettuce bolts (goes to seed and gets bitter), so I've learned lettuce is a waste of space for me. Chard is pretty, but I don't often eat it. But I eat bagsful of kale all year round, so any kale is an excellent use of space in my garden. If I could grow only one vegetable, my choice would be kale because it's productive year-round and I never get tired of eating it. It's also one of the most nutrient-dense foods, and I can pick exactly the amount I need. (Or rather, instead of buying a small bunch at the store or farmers' market, I can pick a grocery bag of kale.)