Oooh ooh, teacher (waves hand furiously) I know this one! I got a tonne of DVDs, and always jump at the chance to push new flicks on to people.
Horror, as the good person above said, is subjective. Comedies can be classics (Blazing Saddles, Kentucky Fried Movie), musicals (Singing in the Rain) or teen genre, or even based on a certain director/producer' oeuvre, your Apatow's, Mel Brooks, Monty Python, Edgar Wright etc. Horror's pretty much the same, I guess. Check out the list below - apologies to other Horror fans as these titles overlap their categories....but meh.
PG13 Fun - Some good chills, usually higher budget fare - these flicks may have a few jump scares, but it's great if you want a movie for scaring your little brothers and sisters, without teaching them what drugs and boobs are - Insidious, Poltergeist, the Ring, Signs/ Sixth Sense, The Others (if you don't know the twist, do check it out), and Jaws.
Genre Mashup - An American Werewolf in London, Shaun of the Dead, Dog Soldiers, Predator/Alien (only fragments of Horror in these movies, but I argue them in nonetheless) Pan's Labyrinth, Black Swan, Repo the Genetic Opera, Army of Darkness
SFX/Gore/T*tties - If you love movies over the top - check out Piranha 3D, Friday the 13th(any of em), Hellraiser 1(gore, gore gore) The Thing, Hostel series/Saw - Dawn of the Dead (the original if you can, though the remake wasn't bad)
Director schtick - Peter Jacksons Braindead (Dead Alive) or Bad Taste - Dario Argento's Suspiria, Tenebrae or Phenomena, Kubrick's version of the Shining, Sam Raimi's Evil Dead - John Carpenter - Halloween, David Lynch...just David Lynch.
Cult Collector - Devil's Backbone, REC (don't watch Quarantine if this available instead - REC has a way better ending) In the Mouth of Madness, Sam Niell in Possession (rare 1981 flick), Jacob's Ladder - and definitely Kill List (it takes a while to get going, mind you) Undead is goofy fun
Foreign Collector - er REC, Ringu, Devil's Backbone - but also Ichi the Killer/ or Audition, a Thai film The Meat Grinder, Inside (creepy French film), Martyrs (really creepy French film), Let the Right one (not very scary, but a great vampire movie)
Horror TV - American Horror Story, Twin Peaks, Walking Dead (haven't actually watched this one yet - but heaps of people recommend it

, Tales of the Crypt, Twilight Zone and the X Files - worth checking out at least, if you ever get the chance to borrow a boxset of someone.
Video Nasties - umm just Wiki that term - the list pushes the R-Rating / X-Rating line to say the least.
Personally, I'd always say The Exorcist as the best horror movie, but not everyone gets spooked by demons. Youtube some of the trailers above and see what grabs you.