The official language of Extreme Programming. Because it makes refactoring as painless as possible.
For people who like visual IDEs, the gold standard is always a Smalltalk environment.
Created by Alan Kay and others. See this Dr Dobbs Journal article http://www.ddj.com/documents/s=7119/ddj0205a/0205a.htm
Dan Ingalls on the Design Principles (from 1981). The purpose of the Smalltalk project is to provide computer support for the Creative spirit in everyone. Our work flows from a vision that includes a creative individual and the best computing hardware available. We have chosen to concentrate on two principle areas of research: a language of description (programming language) that serves as an interface between the models in the human mind and those in computing hardware, and a language of interaction (user interface) that matches the human communication system to that of the computer.