Khumbu Icefall.
For the number of people who've been killed there, versus the number traversing it, it has to be right up there.
Shipton, on his Everest Reconnaissance expedition in 1951, explored the approaches to Everest thoroughly. He dismissed the Khumbu Icefall from consideration as an approach because of the obvious danger. To put that in context, Shipton and Tilman pioneered the ferociously difficult approach to the inner sanctuary of Nanda Devi. Shipton led the 1935 British Everest expedition from the north.
Over the decades I've personally known two people who died in that icefall. Until you've been in an icefall at that latitude you don't understand how unstable they are compared to icefalls in places like Denali, in the subarctic.
It is used simply because there is no other alternative. Experienced mountaineers consider it the most dangerous section of that route, and try, if possible, to traverse it as little as possible, preferably at dawn.