Well, some sort of props must go out to barley and hops!
But my favorite plant in the whole world is nettles. Yep, that’s it. Stinging nettles. Urtica dioica. It’s yummy steamed with garlic, it’s medicinal (wonderful strengthening and cleansing tonic for the liver), it’s great to wield as a whip, chasing your brothers around the yard and stinging them. But my favorite thing about the nettle is that there’s an antidote to the stingers inside the plant. If you get stung, you should pick some leaves and stems and crush them, mash them between two stones or something. Then place the goo on the sting. It will take the sting right out!
I think there’s some sort of Moral Lesson in there, but I’ve not yet decided what it is.
(Dandelion is a runner up, just because it’s so damned useful and pretty, sitting there humbly in your yard just begging to be used in salads, steamed, wine, tea, fried dandelion flowers, etc. Passionflower’s my favorite plant that looks like it comes from outer space.)