After reading John Dougherty’s answer on Quora, I was tempted not to answer. But, that was only a fleeting thought.
It is hard to limit it to just one language. I was originally thinking of Ada. It was a structured language designed by committee. It came out when our team was working on SmallTalk at Tektronix. We were looking at various languages to put on our new workstation and Ada was one of the languages we studied. It was a good language but did not have an Object-Oriented component at that time. It has since added that feature, but it is a language I do not see too many people using any more. It was based on Pascal, a language that was used at Universities to teach students programming. It also is rarely used. But they all trace their ancestry back to Algol, another language designed by committee. It was a great language except for nobody being able to agree on the format for input and output commands. Algol’s high point was its use as the low level programming language for Burroughs computers. Burroughs systems were designed around Algol and when the Army asked Burroughs to write “low” level programs in assembler, they had to write an assembler interpreter in Algol. Good language, any structured language can trace its roots to Algol. This includes C, C++, Pascal, Ada, and Java. One of the best languages no one ever uses.
The language I would like to give honorable mention to is LISP. It was invented about the same time as FORTRAN and COBOL and was really popular in the Artificial Intelligence Community. It was the first language I learned after writing two major projects in FORTRAN. It is quite different than any other language using structures within parenthesis for both its programs and data. It made a comeback in the 80’s with Common Lisp and has had some derivatives that are reasonably popular. But of all the languages I have programmed, it remains the one I enjoyed the most.