Microsoft Word 5.1 on the Macintosh was published in 1992 and was generally seen as the best word processor ever invented. It's UI was perfect. It won a lot of praise in the press, all of it deserved. Sadly, after version 5.1, Microsoft began to focus on the integration of the Office Suite, which meant re-doing all the apps so that they'd be open to programming. And more features got added, and the UI became cluttered. For those of us who sometimes use Word, but don't need the rest of Office, Word 5.1 remains the high point.

Also, as others often said, 5.0 seemed like a broken release, and the ".1" was a much bigger upgrade than is normally implied by a non-major-version number. So it is version 5.1, and not version 5.0, that is remembered fondly.