I'd say skeletons.
They give you bones for bonemeal, which can get you seeds, bread, flowers, tall grass and if you have just one of each mushroom you can create lots more by making giant mushrooms and cutting them down, giving you tons of mushroom stew.
They also give you arrows and now even bows. (with a rare chance they are enchanted too)
Skeletons as they drop free arrows, bows now, and bones for growing trees/food. Also mooshrooms are pretty on par because of steak, free mushroom stew, leather, and mushrooms if you shear them, also are much easier to handle once you find them.
As most people have been saying, Skeletons and Cows (and by extension, Mooshrooms).
I personally do not kill Cows, as Leather can only be used for weak armor, Beef is not that much better than, say, Bread or Fish, and not killing a Cow means I can milk it, and Milk is a VERY important ingredient for Cake. I do not shear Mooshrooms either, as I'd most likely already have Red Mushrooms if I'm in a Mushroom Biome, and I can use a bowl on them to fill the bowl with fungally-infected milk, AKA Mushroom Soup.
Spiders. They provide string. But string can be crafted into wool, which makes beds, a fishing rod will give you food, as well as a means to kill mobs (look it up if u don't know) and they are able to give you string for bows. Thats beds, a food source, and an unconvential weapon, as well as a conventional one.