I think Vista was the least popular as you say, but I think that was because the upgrade path/incentive wasn't there. The upgrade from XP to Vista meant a lot

I think Vista was the least popular as you say, but I think that was because the upgrade path/incentive wasn't there. The upgrade from XP to Vista meant a lot of software stopped working, especially from a driver perspective. If you're using some niche software that "just works (tm)", why change? Especially if it costs a lot of money to upgrade or your system isn't networked. The UK tried to upgrade its XP running backbone years ago, failed - and still got billet £10+ billion.