Unetaneh Tokef Kedushat Hayom. Behold the holy power of this day, it is awesome and full of dread.
If you were Jewish, attending Yom Kippur services is the one thing you do not skip out on. Period. The point I stopped going to Yom Kippur is about the point that I realized I wasn't really Jewish anymore at all.
That's always been my favorite part of the service, along with Kol Nidre.
I always had an issue with Unetaneh Tokef. It implies predestination, which doesn't seem very fitting with the religion otherwise.
Kol Nidre is fascinating. The prayer begging God to release one from promises one was unable to keep, it is a favorite of anti-Semites saying that Jews don't have to keep their deals.