Just realised... mcedit's more accurately the longest used, at least in terms since first used [maybe ~ unsure], and would (and do) still use. But then, if longest used is counting actual time used... nnnnnnothing comes close to emacs. No other text editor serves also as my irc client, organiser, therapist, games system, rss feed, and potentially oh so much more, like email client, 3D modeler (no, seriously), video editor, data analysis, file management, graph generation, slide-show presentations, astrophysics stuff, drone piloting, publishing preparations, printer operation, 3D printer operation, databases, finance and transaction handling, accounting, audio editing, screensaver, web browser, even potentially replacing my terminal multiplexor and even my window manager.
But I did start using mcedit a clear decade before emacs. ... I probably launched each for the first time, around the same time late 2003 to early 2004, but didn't start using emacs (at all) until 2014, whereas mcedit soon became a fave (especially in the midst of the nano'ites thinking they had won their category of text editor, yet few of them seemed to have even heard of mcedit).
... Ah well, even though it's still legit, if I had thought of it at that time, I could have wrangled mcedit into longest, leaving room for something else as honourable mention. ... Maybe neovim. Maybe leafpad.