I worked for IBM on OS/2 Warp and it was a pleasure to work on. I wrote my first multithreaded console program on OS/2 1.0 (which had no GUI; Presentation Manager didn't come along until OS/2 1.1)

By the time OS/2 Warp came along, it was so far ahead of Windows 95 (parts of which still ran in 16 bit) that it wasn't funny. There were DOS games you could play in OS/2 that you couldn't play in Windows 95. But Microsoft just had that overpowering marketing campaign ("Start Me Up!" etc). Windows did eventually catch up however (and pass it actually) when Windows 2000 was released.

Some additional OSes that one might consider checking out if you're interested in that sort of thing are Digital Research Concurrent DOS, its GUI (GEM), BeOS (which is the best multitasking/multithreaded OS I've ever seen, and would be powering Macs and iPhones to this day of Jean-Louis Gassee hadn't been so greedy when Steve Jobs tried to license it), NEXTStep (which went on to become the basis for modern Macs and iPhones), and DeskView/X (which was another amazing OS that was light-years ahead of Windows at the time of release).