Six and Twelve, a nondescript soup and sandwich place on (of course) Sixth Avenue and Twelfth Street, makes an incredible miso soup that is hearty enough to be a meal in itself. You have to ask for it, though; it's not included in their rotating selection of six daily soups (also superb, for the most part).
The miso soup has a rich miso broth base and is positively crammed with grated carrot, fresh spinach, shittake mushrooms, and large chunks of tofu. There are almost more vegetables than soup! It costs $2 for a small cup and $3 for a large. The Korean owners will prepare it right in front of you, in a little tin pot on a gas range in the back of the store, where they do all their cooking, with great skill, mindfulness, and care.