Albus Dumbledore: the corpse of his sister Ariana Dumbledore.
Seamus Finnigan: A banshee.
Hermione Granger: Professor McGonnagall telling her that she failed her exams or failure in general.
Neville Longbottom: Severus Snape.
Remus Lupin: the Full Moon.
Harry Potter: a Dementor.
Parvati Patil: a mummy.
Tom Marvalo Riddle: His own dead body.
Dean Thomas: Disembodied living hand.
Molly Weasley: Dead family/friends.
Ron Weasley: A giant Spider (or any spiders at all).
Padma Patil: a giant cobra.
Fred and George Weasley: Lord Voldemort.
Justin Finch-Fletchley: Lord Voldemort.
Dobby: Lord Voldemort.
Katie Bell: Lord Voldemort.
Severus Snape: Harry Potter lying dead (in the fear of his betrayal of Lily is complete).