I think I would have to say rabies is the most dangerous because even though we have great treatments for it and the vaccine is almost 100% effective it still claims a lot of lives each year.
Rabies is particularly dangerous in the poorer parts of the world where effective treatments are not available and often in these areas bats can pass the disease onto domestic animals like dogs who then pass it to people. Each year rabies kills more than 20,000 people but we have no way of knowing how many cases can be directly traced to bats.
The next big one that we think bats are involved in the spread of is Ebola virus this is quite new research and there are teams right now working in Guinea and other west African countries to see if bats could be spreading Ebola. We know that bats can carry it and we know that they can get Ebola and recover from it but we are not sure if they are the main spreaders of the disease yet. Ebola is >60% fatal and with some strains it is >90% fatal even with medical treatment as we have no cure and treatments are quite limited.
Probably the most high profile disease linked to bats has been the SARS (Severe Acute Respiratory syndrome) outbreak in 2003-2004 this was worrying because it spread so fast so far and it was very infectious . Eventually SARS was contained mostly through quarantining of patients quickly and really good work by medical staff. Now with the spreading of MERS (Middle East Respiratory Syndrome) virus we are using alot of the same things we learning from SARS and hopefully this will help to contain it and stop so many people from dying.