Kale gets so little love. It's treated as decorative plant fodder on vegetable trays, planted like a wayward herbaceous border in gardens, and generally ignored, abused and unloved. It's a shame because all the varieties of kale are chock-full of vitamins and minerals and are a dietician's dream.
Kale also tastes great. In the pantheon of dark-green leafy vegetables, kale doesn't have the sharp, wasabi-like tang of mustard, doesn't have the bitterness of chard or broccoli rabe, and has a more assertive flavor than beet greens. It's not nearly as fibrous and tough as collards, either.
Black kale, called cavolo nero in Italian, is probably the richest-tasting and best-for-you variety of kale (feel free to disagree). It's a hardy winter green, which means it's in season right now, and the longer it sits in the ground the better its flavor gets, within reason.
As with any green, don't accept black kale with discolorations, spots or any kind of wet mushiness whatsoever. When you pick up a bunch of kale by its stems, the leaves will flop over even on just-picked specimens (due to the weight of the leaves and the relative thinness of the stems), but avoid obviously badly wilted kale.
Kale tastes great in soups; the simplest recipe for caldo verde, the famous Portuguese soup, is Portuguese sausage such as linguiça or chouriço with broth and shredded kale. And of course it can be braised or gratinéed like any green, but the best thing to do with kale is roast it.
Yes, roast it. Preheat your oven to 400ºF, toss a pound of very well-washed and dried kale with a scant spoonful of olive oil, and roast it in one layer in the oven until it turns crispy and the edges just start to brown, 15 to 20 minutes. As soon as it comes out, scatter salt on the kale. (Don't salt the kale first, or the salt will draw water out and you'll have mush in the oven.)
Black kale works best for this because its leaf shape holds together best; it eats most neatly, and because it's ever so slightly sweeter than its curly cousin when it goes into the oven, it's even sweeter when it comes out. Mature black kale, baked in the oven, can be used to scoop light dips, though that way lies caloric ruin for a great, healthy alternative to potato chips. (You're out of luck on the guacamole–sorry.)