Collards are the toughies of the crucifer greens. They're thick, dusty green leaves, broad and long. We have collards much of the late summer. They're a little more bitter than Kale, and are rarely eaten raw. They're usually steamed or stir fried, served with vinegar.
Mustard greens are sharp and tasty - with a real bite.
I love them added to a salad - hidden treasures of flavor. They're more tender, are available much of the summer, and can also be steamed, stir fried, or added to casseroles. Beet and Turnip greens are the wonderfully nutritious 'extras' that grow atop their more familiar root vegetables. Both are available in the beginning of summer, are very tender and green tasting - generally mild flavored, excellent added raw to salad, steamed for casseroles or eaten with vinegar. Swiss Chard and Spinach both need an honorable mention here - though not in the crucifer family, each is high in vitamins A and C, and have good portions of minerals.