I don't think I really have a least favorite OS TBH, I've used various OS's throughout the years nowadays and most of them are just fine for what most people do.
I think the OS I've had the most problems with though is probably macOS, but that's only because the majority of my experience with it has either been on old ass Macs or via hackintoshing, each which have their own quirks and issues, but those are less related to the OS and more related to the hardware and how only Apple products can officially exist in their own little bubble.
Vista I also don't have a problem with, because the majority of the issues people had with Vista were not OS-only related so much as they were with hardware manufacturers that A) sold "Vista capable!" PCs that were not, in fact, Vista capable whatsoever and B) Couldn't be assed to create new drivers for the new OS, which caused stability issues. I suppose you could argue high hardware requirements being a problem but really beyond that, some bloat, and the usual launch bugs literally every OS faces the first year, there was nothing overly wrong with Vista beyond perhaps being different.