Answer by Tom Smith:
I'd vote for Professor Robert Dewar of the Courant Institute of NYU.There may be others but I'm restricting myself to people that I've actually met and worked with. It's all to easy to vote for people you have never met.I'd agree that Stallman must be a genius but I've never met him, anyway Dewar was a genius, just look at his history here:Robert was a true renaissance man, he turned his hand and brain power to many subjects, languages, teaching students, formal language theory, open source etc. We was also a great musician and even built a small stage in his penthouse on 5th Avenue in NYC. He could also make as much money as he wanted to. He invented gnat, an Ada tool chain built on top of gcc and set up a successful business to support and develop it, AdaCore.He could be breathtakingly smart. For example, I worked with him on the Ada threading model. A problem with the model was that it wasn't symmetrical and if the theads arrived out of order the running thread might have to start the waiting thread and then yield. This was very slow! Dewar came up with the idea of thread borrowing (he coined the name on the spot too) so that the stopped Ada thread took over the running hardware thread and the other Ada thread stopped. This meant that you could save a lot of context switching. I implemented his idea that afternoon and we had the thing running the following day. The speed was phenomenal, I could hardly believe it worked, it was magic.Dewar also wrote a COBOL compiler in COBOL, this was the Realia compiler and is still used today. He could have retired on the money this generated! The compiler was remarkable because it was designed to have no limits. You could have 10,000 parameters or 8,000,000 lines of code, didn't matter. Can you imagine writing a compiler in COBOL?One of his many hobbies was playing the frequent flyer programs of the big airlines and would often fly to Europe 'the wrong way round' because he'd worked out some round the word ticket.So he was quite a guy and my life is a little better for having met and worked with him.