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This is a pretty though decision, as I think the Internet and the Blockchain are fighting over the first place. However, we do have more or less 82 years left to go - so there might be an even bigger and better invention which, according to our current state of knowledge, we cannot even imagine. Of course there will be some big inventions in the AI sector - considering that every year the computing power almost doubles. With bigger and better computers we are able to have faster and more precise neural networks. 

In fact no one can predict what the future will hold for us.

I would say smartphone is the best invention untill now for the 21st century.

The mobile operating system is the best invention of the 21st century. There are many notable 21st century inventions owing their success to humans having the power of a computer at our fingertips. Those inventions would include social networking, youtube, and to  a very large degree the Internet of Things. The speed at which information flows now is astonishing and it is through the mass, mobile connection of humans made possible by the mobile operating system,