Ken Thompson has a unique place in the pantheon of great programmers. Android as a linux variant is a direct descendent of UNIX which Ken wrote with Dennis Ritchie. The vast majority of systems code is written in computer languages derived from B, the language Ken created that was the basis for Dennis' C language. If your human language needs more than ASCII can handle, then you're probably using UTF8 under the hood every day. Guess what, Ken invented that too.

Ken's contributions, more than anyone else I can think of, were fundamental and yet so practical and timeless they are still in daily use. Richard Stallman was an important developer and is an even more important evangelist for freedom. Linus Torvalds' open source creations linux and git have spawned billion dollar companies merely to support them. Ken has out innovated them handily. Don Knuth is a great artist, and thinker whose ideas are in daily use. His Tex and Metafont are at the core of the publishing industry, but most people don't come as directly in contact with his work as they do with Ken's.

For this reason, Ken eminently deserves everyone's vote as the best programmer of all time.

I just learned that Ken's game Space Travel was the impetus for him to create UNIX in the first place. Here it is in Dennis Ritchie's own words:
The Space Travel game