I would probably have to say that Ebola is probably the worst, most contagious fast-acting deadly disease. As I understand it, less than 10% of those who get Ebola survive. Even the most deadly influenza and bubonic plagues left 25% or more alive.
I came to this thread expecting to see people talking about AIDS and I was going to say Ebola, but I got beat to it. I remember watching a movie about Ebola when I was younger and I'm surprised more people don't know about the disease. Those inflicted could die in 2 days of symptom onset.