Being a huge fan of the space program I would say

1> All time greatest achievement Landing on the moon

2> 20th Century's greatest achievement Landing on the moon

3> So far the greatest achievement of 21st Century None

4> All time worst achievement The Holocaust

5> 20th Century's worst achievement The Holocaust

6> So far the worst achievement of 21st Century That we still live blindly to our surroundings and are so easily lead astray, even with all of the new technological advances we have that give us so much more information than we ever thought we could get.