I looked at the list a couple of times and I think that Voldemort's return was the scariest.
The fight in the Department of mysteries seemed more of an action sequence to me as I read it. It was definatly faster paced.
The dementors are scary too but the sheer discription of Voldemorts rising was to me the scariest thing she has written yet.
This could be a classic film moment if they do it properly.
The way Harry kept praying "Let it have drowned, Let it have gone wrong" Then as Voldemort rose up out of that steaming cauldron I could picture it quite vividly.
The thin man stepped out of the cauldron, staring at Harry... and Harry stared back into the face that had haunted his nightmares for three years. Whiter than a skull, with wide, livid scarlet eyes and a nose that was flat as a snake's with slits for nostrils...
Lord Voldemort had risin again.
*shivers*