I'm with the first post. A baby crying. Especially when it's constant and it sounds like they're really in pain. I hate hearing that noise because I want to help the child but I feel so helpless! (i'm not a mother by the way)
2nd worst noise is the sound of snoring right next to you.
baby crying nonstop
motorbikes, esp. when they make all that noise just to show off
the blender (even worse when someone else is using it)
howling cats at nightjazz and rap music