By going by sheer numbers alone it would be the battle of Kiev, the Soviets lost 700k men out whom 600k were captured. An entire army group ceased to exist in the matter of a month, the Germans only lost 40000 men in exchange.
At Stalingrad the Axis also lost an army group and nearly 800k men but it was in the course of 6 months and the soviets lost 1.1 million men.
There's the battle of Changping also where Qin burried 400k Zhao troops alive but however lost 250k men.
Going by ratio of casualties and lopsided victories contenders would be the Battle of Chaeronea where Sulla crushed a 120,000 Pontic force at the expense of a few hundred casualties, Minsk where the Germans captured or killed 400k Soviet troops at expense of 12k losses, the battle of Cannae where Rome suffered its greatest defeat ever when it lost 85k troops to Hannibal while only inflicting 6k losses, Cerami where 160 Norman knights killed the bulk of a 300-10000 strong Arab army among many others
If we were to go by strategic or historic consequences then in my opinion the most crushing loss of all was the Byzantine defeat at Yarmouk which led to the Islamization of the Middle east and the effective evaporation of Christianity outside Europe. There's the battle of Cajmaraca which saw the Spanish take over the Inca Empire and by extension South America, Manzikert which saw the Turks take over Anatolia and the Sack of Baghdad by Mongol General Hulagu Khan
Source: reddit post