Without a doubt, the answer has to be
CYAN WOOL CYAN WOOL CYAN WOOL
I mean, how can we live without it? And don't say you can, be that is an obvious lie!
I believe that the hands down best block in minecraft is as follows:
net.minecraft.block.Block
It is every block and no block at the same time. Every block in the game will exend this class, making it the root of all we hold dear about Minecraft. It is Cyan Wool, It is Diamond block, it is dirt, it is stone, it is EVERYTHING. Clearly there is no other block in the entirety of the net.minecraft package that is its equal. Without net.minecraft.block.Block, Minecraft itself is nothing.
net.minecraft.block.Block