This is a hard question to answer. All cancers can be deadly. It depends on the type of cancer, the location, the Metastasis etc. It also depends on the persons general health and the stage of the particular cancer.
Testicular cancer is one of the easiest to treat, even at stage 3c. Testicular cancer, even at stage 3c has over a 72% cure rate. Testicular cancer at stages 1 and 2 are over 98% curable and combined, testicular cancer has over 95%
Pancreatic is the most deadly and hardest ot treat. There is pretty much nothing they can do for someone with pancreatic cancer. The survival rate is very low. About all they have been able to do is extend the life of the person who has it via Chemo and radiation. There have been a few cases of people with Pancreatic cancer that have lived for a significant amount of time once diagnosed, over 10 years. However, the death rate related to Pancreatic cancer is extremely high..
Now, Lung cancer and Breast cancer kill more people every year more than any of the other cancers combined. But both of these can be treated and the prognosis can be very very good and they have validated treatments for this. But here is the problem, once either of these cancers reach stages 3 or 4, it becomes a problem of not curing, but extending the persons life. These two cancer are very curable when caught early,,,but as with most cancer, most people find out about accidentally because they were not looking for cancer, so the later you are diagnosed, the worse the survival rate