So I saw one topic the other day that said "What is the most useless item in Minecraft?" sure enough a lot of people replied. But what I want to know is, what do you think is the most useful item in Minecraft? If you care to explain why, do it.
It's pretty much the base material for everything in Minecraft. If the thing you want doesn't include wood as part of it's recipe, then wood will almost always be required somewhere along the line to obtain that thing. Even something as simple as Mushrooms require wood for the bowl so you can eat them.
Obvious exceptions to the "requires wood" list would be anything you can gather from the world with your bare hands. But frankly, that's not much, and most of that has very little practical use. Even an Apple requires a tree (made of wood) to drop it.
Wood is the single most important and most useful resource in Minecraft.
It has aesthetic value, acts as an infinite source of wood (required for every tool and useful for housing), is accessible from the moment you find wood, you get it at the same time you find wood, and can be burned as furnace fuel in a pinch. It's kind of like wood except it has more uses and takes more time to make use of.
I'll agree with Kholdstare (and generally everyone else who said wood), except to specify "Oak Sapling"... which will generate apples for food in addition to an endless supply of wood (to make planks, slabs, stairs, doors, sticks, wooden tools, etc).