This is probably the easiest to build tnt cannon, and by far the most effective, as you can easily stretch it as wide as you want, and have all the tnt fly at the same time.

Here, you will use a tnt cannon with the touch of a button.
To refill it, put tnt underneath the obsidian pieces with the redstone on top. The redstone will not be affected by the explosion.

NOW BLOW UP THAT NPC VILLAGE! :D