One of the worst Crusades was the Fourth Crusade, which took place from 1202 to 1204. This Crusade was originally intended to retake the Holy Land from the Muslims, but it was diverted to Constantinople, the capital of the Byzantine Empire. The Crusaders laid siege to the city and eventually sacked it in 1204. The sack of Constantinople was a brutal event that resulted in the death of thousands of people and the destruction of much of the city's art and architecture.
Another particularly violent Crusade was the Albigensian Crusade, which took place from 1209 to 1229. This Crusade was waged against the Cathars, a Christian sect that was deemed heretical by the Catholic Church. The Albigensian Crusade was marked by extreme brutality, and it is estimated that tens of thousands of people were killed.
Overall, it is difficult to say which Crusade was the worst, as they were all characterized by violence and bloodshed. However, the Fourth Crusade and the Albigensian Crusade are often cited as being among the most brutal and destructive Crusades.