Turbo Vision/Turbo Pascal for MS-DOS. Borland put together one of the best instances of an object oriented library that just worked. The follow up with Delphi for Windows was the most productive environment I have ever experienced, until I was priced out of it in their pivot to "Enterprise" customers.

Nothing since is anywhere near as productive. Lazarus is ok, but the documentation is horrible, (almost non-existent, only doc-strings in many cases) which makes the system far less useful.