What is the greatest power or weapon in possession by humanity?
Metaphorically, knowledge.Realistically, atomic weapons.
The human mind,the man behind the ideas.
Nuclear bombs such as the tsar bomba. It was a Russian bomb tested in the cold war and was equal to 100 megatons of TNT. If you watch a video on youtube the cameras where 60 miles away and the bomb was so bright the camera went solid white.
the nucular or atomic bomb
If your talking about an actual weapon, it would be a matter/anti-matter weapon.
The Greatest is Nuclear atomic Weapon. It is consider worldwide as the worst weapon in humanity.http://news.bbc.co.uk/onthisday/hi/dates/stories/august/6/newsid_3602000/3602189.stm
your tongue, with it you can bless or destroy