One of the most maddening things about cancer is that, in a sense, there is no such "thing" as cancer - cancer is more a category of diseases, all of them caused by certain genetic mutations in cells. For this reason, some cancers are deadlier than others. However, even on a statistical level, it is difficult and not necessarily accurate to discuss which types of cancer have the worst prognosis. Overall, pancreatic cancer has a 5-year survival rate of just 5%, making it one of the worst - but, as always, statistics do not necessarily tell the whole story.
Before discussing which type of cancer has the worst prognosis, then, it is important to understand the difference between a prognosis and a statistic. Statistics are everywhere when it comes to cancer - 5-year survival rate, 10-year survival rate, treatment response rate, etc. However, none of these are actually prognoses. A prognosis is a doctor's judgement about an individual case, at least part of which rests on highly individual factors like certain risk factors, age, and the extent of disease progression on diagnosis. Cancers which kill large numbers of people can actually be curable in individual cases, especially if they are caught early in young, otherwise healthy patients (who typically have the best outlooks).
Just one simple distinction between different cases of the same cancer - the stage, or extent of progression - makes an immense difference. According to the American Cancer Society, small-cell lung cancer survival rates range from 31% in Stage I (the earliest stage) to just 2% in Stage IV. Some cancers, like lymphoma, may be curable in their early stages, but incurable in their later stages. This is why doctors often emphasize how important it is to catch cancers as early as possible - simply put, in almost all cases, the earlier they are identified, the more effectively they can be treated.
Second, the types of cancer with the worst prognosis are not always the types of cancer which are deadliest. Some cancers have extremely high mortality rates, yet seem to kill relatively few people because they are so rare. Other cancers are more common and very deadly. Lung cancer both has a poor prognosis and is the leading cause of cancer death - but, outside of smokers, it is a comparatively rare disease.
With all that in mind, we can make some statements about the cancer with the worst prognosis in general terms - by which we mean, the cancers which kill the largest number of affected people. Pancreatic cancer is generally considered to be the deadliest cancer in terms of overall prognosis. Only about 5% of people diagnosed with pancreatic cancer will survive for 5 years after being diagnosed - although, even in that case, the survival rates range from 37% for early Stage I cancer to just 1% for Stage IV (in this case, part of the reason the overall survival rate is so low is that most people are not diagnosed until the disease is very advanced). The survival rates for the most common aggressive form of brain tumour, glioblastoma multiforme, range from 14% in the early stages to 0%-4% in advanced stages.
Several other cancers also have extremely poor prognoses, meaning 5-year survival rates of about 5%. These include esophageal cancer (which, again, is usually not caught in its early stages), small-cell lung cancer, and liver cancer.