What are some of the most dangerous viral disease?

Dangerous viruses. Are you referring to most virulent virus, fatal rate without or with treatment or most deaths historically? Dangerous viruses include: Marburg Virus, Ebola Zaire, Hantavirus, H5N1 (Avian Flu), Junin Virus (Argentine Hemorrhagic Fever), Lassa Fever, Crimea – Congo Fever,Machupo virus (Bolivian hemorrhagic fever), Kyasanur Forest Virus (KFD), Rabies, Smallpox & Dengue fever.
A long list. Almost any virus in the right circumstances can cause severe illness and sometimes death. The obvious dangerous ones include things like rabies, ebola, Marburg virsus, West Nile, Chichungya, enchephalitis and a host of others. Some are more dangerous in the already sick or immunosuppressed such as Herpes and Ebstein=Barr virus.
A few favorites: Ebola, marburg, rabies, smallpox, and lassa fever, to name some of the viruses you'd least want to have.