I am opening this
section to answer multiple requests to include some people that I
stubbornly refuse to consider visionaries.
If you have good arguments to the contrary, I will gladly listen to them.
His older works on formal grammars were revolutionary, but his recent theories of the "Language Organ" seem unsubstantiated, and his political criticism, while undoubtedly guided by good intentions, doesn't seem grounded. I attended his talk once, and listened to a rather dull list of somewhat exaggerated criticisms of corporations, without any understanding of their absolutely essential role in the modern society (how can you run a complex system without aggregating agents into task-specific groups and allocating resources for their activities - every other system, from ecosystems to organisms to computer programs does that!). In addition, he refused to either suggest any scenarios of the future, or offer any suggestions on how to fix the situation. Some visionary...
You can read his books and make your own conclusions.