The worst disaster in space history was the Nedelin catastrophe which also took out some of the USSR’s top minds.
The second worst disaster was the Plesetsk disaster, also by the Soviet Union.
Not sure what metrics you’re using for that claim.
It’s unclear if the Nedelin catastrophe should count. That was done by the military rocket program and not the Soviet Space Program. It did take place at the Baikonur Cosmodrome which is the same place the civilian space program launched it’s rockets from. If you include the Nedelin disaster then you have to include the non-NASA/military rocket disasters of the US. I know the US had at least 2 ICBM disasters with heavy loss of life.
Plesetsk counts because for some reason everybody decided that launching spy satellites is the job of the civilian spaceflight programs.
