Windows 3.x: 8+
It was miraculous in its day. After years of using DOS and a variety of clunky task-switching programs like DesqView I was thrilled to have a GUI and a real memory manager. From a tinkerer’s point of view it was pure gold (Brian Livingston’s original Windows 3.1 Secrets was roughly 1200 pages.) Windows for Workgroups 3.11 introduced TCP/IP support, and even included a network card (I think I still have the little Microsoft-logo’ed screwdriver that came with it). With Norton Desktop for Windows or PC Tools, you could have a shell that foreshadowed the full-blown GUI of Windows 95.