Actually NT 3.1. 3.5 followed, then 3.51, then 4.0, then 5.0 (2000), and then the NT line ended as it was unified with the non-NT line.

Technically 9x line ended since Windows XP was NT-based and not 9x-based.

Yes, I suppose so. I didn't want to say the 9x line ended, because it's really the line of DOS-based OSes, and while the NT line is ongoing, it's no longer called NT. 2000 was the last version to mention NT, and it wasn't part of the name itself, just a tagline.