I guess it depends on your definition of worst.
I would think HIV/AIDS for sheer loss of life and potential loss of life in the future, plus the tie-ins with with economic issues, leaving a generation of orphans, and relationship to increased occurrence of other diseases (e.g. multi-drug-resistant TB). Despite the vast amt. of research going into HIV treatments, at the heart of the problem is that HIV (like many infectious diseases) disproportionately affects the world's poor, and good science is not sufficient to tx it; there has to be a combination of education, prevention, and cooperation b/t nations, drug companies, scientists, and NGOs to get
affordable,
practical treatments to the places that are most affected. Sure, people who can get optimal tx might live for decades, but the multiple drug cocktail, take with food, must refrigerate kinds of drugs are not gonna solve the problem in Sub-Saharan Africa. For this reason, and because HIV is complicated by other issues of poverty, corruption, and war/unrest, I believe that HIV/AIDS will continue to be THE major public health problem on a global scale.
Which is the worst to have? My vote goes for Ebola. Up to a 90% mortality rate, and you die bleeding from every orifice, with your organs liquifying, spewing up black vomit, along with other nasty side-effects like sloughing of your gut. But as
LizzyM pointed out, it is really "too good" at its job, and is not an efficient pathogen because it kills too quickly to spread to epidemic proportions.