The catch is it has to be a single file and no dependencies other than what the OS of its day provided by default. I am sick of OSes today shipping with hundreds of thousands of files, on install DVDs. I am looking for the most useful yet tiniest ever. Small is indeed beautiful (and this program's chief architect should be President).
My choice: Microsoft Word for DOS. Specifically, version 5.0a, 622,428 bytes. [Version 5.5, patched for Y2K, is available from MS for free].
Some of the reasons I love Word.exe? Ran native on OS/2, had a shallow mouse-and-keyboard accessible menu tree (that negated the need for obscure WP-like macros or keyboard templates, although it had one of the best keytemps ever), integrated support for a powerful yet readable macro language, RTF support, embeddable images, CR or CR-LF text file support, changeable screen resolutions (including a half readable graphics mode), first DOS application with native mouse support. And practical things like a hefty 8MB file size limit, auto-created backup files, auto-generated "DOC" file extension, automatic on-screen pagination and absolute 100.0% stability. Pity that XP broke the clipboard access...
At one point I worked at a 500 person engineering firm that was still running Word.exe right into the Windows 98 era. I've written applications that depended on automated calls to Word.exe. Low tech, high age friends of mine use Word.exe to write, manage contact lists and convert documents -- for almost 20 years now [Word timeline]. They bought a new printer 6 or 7 years ago and the dealer installed Windows to install the printer drivers. On their next trip to British Columbia, on route to Hawaii, I was called upon to put it back to "just DOS + Word" for them. Word.exe keeps on ticking.
If we allow more than a single file but still limit things to what shipped with the core application, then Word.exe gets even better because of: postscript support (30KB), superb help file (105KB), the most complete spell check database at that time -- that allowed user and global add-on dictionaries (180KB), full thesaurus (320KB), excellent tutorial system for mouse and keyboard (900KB)]. Heck, if you excluded the thesaurus & tutorials you could be fit it all on a single bootable 3.5" floppy.
Word.exe. I don't leave home without it.