Vista
I ran through Vista in about a week. The system sucked. I updated the Vista immediately to Windows 7 in my 64 bit system when I got it and when Win7 was fairly new, because I wanted to head off any future application upgrade issues.
My laptop and "spare" desktop run XP, and work just fine for what they're used for. No Vista there either, thanks.
There was no upgrade path from XP to Win7. You had to either do a clean install or buy Vista, which did have a direct Win7 upgrade path. That sucked.
To the best of my knowledge, you can update to Windows 8 from 7, Vista, or XP. Apparently an upgrade version is only like 40 bucks until sometime in January.