If I were to choose just one veg to grow it would probably be beetroot because it's so easy and versatile, with few pests. It looks great in the flower border and you can start eating the leaves and end with the roots. It also stores well. It even grows ok in containers, doesn't need a lot of water and will tolerate light shade!
Being in the same family as chard it's also extremely healthy from leaves to roots.
I use a rating system to help me decide what to grow and it considers lots of factors, but the main ones are: how tasty it is, how healthy it is to eat, how expensive it is to buy, how big it's harvest is, when it's harvest period is and whether I can buy it organically and if not how much it's sprayed by commercial growers.
Beetroot scores very highly. It's never going to complete with peas or strawberries for taste, but it's still incredibly versatile. A lot of people dislike the earthy taste of the red varieties, but most of them are convinced by the golden ones, which are much milder and serve as a substitute for carrot in spring.
Beetroot is also very healthy, the leaves are particularly valuable, but the roots are great too. The only concern to be aware of is that, like spinach and chard, the leaves are high in oxalic acid. So if you are at risk for kidney stones, eat in moderation.
Although beetroot are cheap to buy, they are also easy to grow and quite productive, so we consider them worthwhile from an economic point of view, but they will never compete with lettuce or kale in terms of 'bang for the buck'.
Finally beetroot is a winter super-star, they keep fairly well in the ground and don't mind a light frost, but they store exceptionally well and we are often still eating them from the store, even after the new season roots are ready.
Beetroot can be grown in containers, but like most things they does better in the ground. It has a fairly deep tap root, so it benefits from a bit of depth (12 wide and deep is ideal). Like many leafy greens it will tolerate light shade, especially in summer.
I try and give my first early crop as much light as possible, but my main-crop sowing benefits from light shade at mid-day.
In ideal conditions Beetroot planted in spring or summer won't go to seed until the following year. However like most veggies, sown too early in spring, or subject to stress (too cold, too dry etc) then it might well go to seed before you harvest in summer.
It's possible to sow beetroot in Autumn in the hope of an early spring crop, but this increases the likelihood that the plants will go to seed in spring, prior to anything other than a leaf harvest. Since beetroot harvested in Autumn happily stores until June, there's really little need to grow an early crop.
Many books and websites will tell you that the best time to harvest beetroot is when it's smaller than a tennis ball, otherwise it will be tough/woody. That's never been our experience and we grow a LOT of beetroot. Woody roots are more a function of time in the ground, than they are size. A 5 month old small root that's sown in February and harvested small in July, is much more likely to be woody, than a much larger 3 month old root that's grown fast over summer (provided it's had enough water).
For more details on the model that I use for describing harvest periods (first earlies, second earlies etc) please see the chapter on my growing framework
The following chart provides an overview of each beetroot sowing to our diet. As you can see we tend to eat less beetroot in early spring when we only have our relatively modest harvests from our first and second earlies. Then as our early main-crop arrives in summer, followed by the storage crop in October. We eat from store from October until the first earlies are ready in late spring.
We don't normally sow a late crop as we need the space for winter crops and we would rather eat from store.