Depends on the platform
The very first was AmigaOS (Workbench 1.3) on the Amiga 500. Then I had a PC with MS-DOS 5.0 (Norton Commander installed on top of that). Later it was Windows 3.11 with newer versions of DOS (up to 6.22). Then Windows 95->98->ME->2000->98->XP->Vista->7 (64bit)
I'm not counting my various forays into Linux.