The basis of most use of computers is the creation of files containing text which may be plain text, computer programs or text with some mark-up language like TEX. To create and maintain such files one needs programs called editors. The two most popular editors that are available on a variety of platforms are vi and emacs. In addition these editors are versatile enough that one can emulate most other editors using the framework provided.
The vi editor is provided along with most vendor distibuted Unix-like systems. However, the GNU project has an editor called Elvis which is compatible with vi and has many additional features. This is an ideal editor for programmers and administrators.
A more general purpose editor is emacs which was created by Richard Stallman-the founder of the GNU project. It is an extremely versatile editor with a large number of configuration files and added packages. Many users familiar with emacs start and end their interactions with the computer with it. It has detailed (hypertext) help and information within it called the info-mode. One can argue that no editor which you can buy is better than GNU Emacs--which can be obtained for free.
A number of other editors are also available in the public archives like Origami (a folding editor) and Micro-Emacs (a clone of Emacs). In fact any user can find an editor to his/her liking among the editors available in the archives or find a configuration of vi/emacs that suits the purpose.