Smalltalk
The language that defined object orientation. Developed at Xerox PARC by Alan Kay, Dan Ingalls and Adele Goldberg through the 1970s; Smalltalk-80 became the version that spread.
Everything is an object — including classes, numbers and blocks of code — and everything that happens is objects sending messages to each other. The syntax fits on a postcard. The system is also image-based: the entire runtime state is saved to a file, and you program inside the live system with a debugger that lets you change code in the middle of a halted call and continue. Alan Kay has said that by "object-oriented" he meant the message passing, not classes and inheritance. The legacy shows in Objective-C, Ruby, Java and in the very ideas of graphical interfaces and IDEs. Lives on in Pharo and Squeak.