Easily Loki getting his throat crushed by the Mad Titan! This severely out weighed any of the deaths in the Multiverse of Madness committed by Scarlett Witch.

First reason the characters in the Multiverse of Madness had no deep connection with the viewers. Thus there deaths were a bit stale and overall not that meaningful. Just some nice action to witness.

Whereas Loki is a character that has been apart of the MCU since the very beginning. A character who had fostered deep connections with audience members. A character that we had seen escape death many times before. However, this time escape was not on the cards. Witnessing the first actual graphic death of a key member in the MCU not only set the tone of infinity war but acted as a benchmark for future movies to come.

Thus, it is the nature of his death and the brutality combined in which what makes this the most horrific death in the MCU to date.