Nasturtium

What a gem of a plant - you don't even have to plant them.
I'm still not quite sure how they manage to get into your garden - surely nobody would need to propagate them from seeds or cuttings. However, once they're there, they not only manage to look after themselves without you having to lift a finger, but they also distribute themselves out to different nooks and crannies and start up little colonies of Nasturtium plants that, with a minimum of oversight and control, can live quite comfortably amongst the plants you're actually trying to grow.
They also reliably produce loads of gorgeous flowers for a decent chunk of the year, which make otherwise nondescript areas jump with cheery flashes of bright red-orange. 
Nasturtiums also make great companion plantss for crops like cabbage, corn, tomatoes, melon, beans and fruit trees. They help these plants by pre-emptively attracting predators and pests such as aphids, drawing them away from their companions. And they seem to be able to manage such pests with far less damaging effects.
To top it all off, you can even eat the bloody things - as garnishes or in a wide range of salads or vegetable dishes. Hence their common names of Indian cress, Mexican cress, or Peru cress. They have a peppery taste similar to rocket.
Unless, of course, you have stomach or intestinal ulcers or kidney disease, according to webmd.com. In which case, Nasturtiums will only make things worse.
But if you don't have any of these conditions, and do eat them; or if you decide to ignore webmd entirely and, like everyone else, reckon they're wholly and safely edible (leaves, flowers, seed pods, the lot - as are all members of the Brassica family), then you get quite a decent dose of Vitamin C for your trouble.
That's very handy, and versatile.
By comparison, the list of downsides is rather brief. Some complain that they need too much supervision to ensure they don't take over parts of the garden. Fine, but it's always a hell of a lot easier to control enthusiastic Nasturtiums than an infestation by any of the many types of weeds that regularly plague our gardens.
Given the choice, Go Nasturtiums, I say.