Currently, it is HIV.
No. Rabies is a virus transmitted by animals and humans to other animals and humans most often through a bite. If the virus reaches the central nervous system it causes severe brain damage and is fatal most of the time.
Trichomoniasis is the most common protozoan STD in the US. Aside from giardia, which is rarely sexually transmitted, it's the only protozoan STD.
Damage to the cerebral cortex is least likely to be fatal; damage to the medulla is most likely to be fatal.
Syphylis
Chlamydia is the most common bacterial STD in the US, and the most common reportable STD. You should know that not all STDs are reportable, so the CDC doesn't get direct reports about HPV or trichomonas.
Itching
HIV
Chlamydia is not the most dangerous STI.
HPV
yes, most likely.
Most cancers are fatal. Actually al of them are potenially fatal.
Most STD's are caused by Bacteria. To give a few examples: Syphilis, Gonorrhea, Chlamydia... (etc)