Most deadly cancer?

Any of several: There are maybe 5000 known cancer types. All will inevitably kill if untreated. Of those that can be treated but not cured, glioblastoma of the brain is especially cruel and kills fast; small cell undifferentiated carcinoma kills about 30, 000 americans a year (basically all smokers) but chemotherapy can hold off the inevitable for a few months. Cures of common pancreatic cancer are rare.

Interesting question: I guess it depends on how you look at it. However, one person - one cancer? As a surgeon the cancer i don't want is anaplastic thyroid cancer. Miserable. Hard to treat. It's not as well known as glioblastoma (a brain cancer) or pancreatic cancer, but it's a very bad actor. I hope yours was a question of pure curiosity. Good luck.