Biggest military upset ever?

[jaichind]

I tend to only count those where both sides had comparable military technologies.  Using that criterion I would say Agincourt in 1415

[jaichind]

In Chinese history, I would say

昆陽之戰(Battle of Kunyang) 23AD which was the founding battle of the East Han Dynasty
https://en.wikipedia.org/wiki/Battle_of_Kunyang

淝水之戰(Battle of Feishui) 383AD which rescued the Eastern Jin dynasty and led to the fall of the Early Chin dynasty
https://en.wikipedia.org/wiki/Battle_of_Fei_River

The main problem of both battles is that the size of the losing army was exaggerated by the winning side for political reasons and on top of that not all the forces of the losing side were actually deployed for battle.  Still, both were for sure considered big upsets even if not on the same scale as claimed by the history books written by the winning side.