The best user interface could also be nominated as the worst: the emacs editor. It's an artifact of the future-past, the editor a sci-fi film from the 1980s might imagine to exist a century in the future.
Emacs' interaction design centers largely on keyboard -- many adherents never touch the mouse in normal use. Its user interface is virtually none at all -- one line at the bottom of the screen for commands and messages and another for the status bar.
This minimalist-maximalist approach gives it a near-infinite feature set with no UI bloat. (There's nothing to bloat.) In 16 years of use I've probably drawn on one-third of its functionality. But I do have some favorites: to cut a rectangular block from the interior of your selection, hit ^x,r,k. To swap two letters, hit ^t; to swap two words, hit alt-t. Alt-/ takes the partial word beneath the cursor and intelligently autocompletes it from the words in all open documents (so a function name like 'validate_duplicate_line_items' can take as few as four or five keystrokes).
The genius of emacs lies in its collection of 'modes', bundles of code that adjust the editor's behavior for the type of file you're editing. Open an html file and the editor will indent and outdent automatically, do syntax highlighting, and enable keystrokes to let you open the current file in a browser or push it to a remote machine. Sure, Dreamweaver and Coda both let you do this -- but those can't provide equivalent functionality (and with the same perfect interface) for plain text, ruby scripts, config files, server log browsing or a hundred others.
Emacs is eminently configurable and extensible: the editor is written in a fast, powerful version of the Lisp programming language. Modes and other customizations don't require some second-class scripting addon -- they use the same lisp language and draw on extensive 'hooks' into the code to alter even core behavior succinctly and safely. Emacs contains a fully-featured web browser, terminal client, mail reader, even a psychiatrist. (Detractors hold that emacs is the perfect operating system, lacking only a decent text editor).
Its downsides are significant. Since its core functions have remained unchanged for three decades, its keystrokes predate all modern conventions: use "^x,f" to open a file, "^k" to cut a line and "^w" to cut a selection of text. Niceties such as dropdown-menus, mouse interactions and context help do exist, but often feel out of place and clumsy. Its look and feel breaks from all modern operating systems.
If you're willing to summit its himalayan learning curve, however, emacs becomes an extension of your fingertips, a frictionless channel from idea into form.
