Hmm, good question... I've watched a TON of horror movies during my life so far...
Best with the most sequels: Friday the 13th! *ohgawdilovejason*
Most terrifying: The Grudge 1-2 (American) *nightmaresforaweekdammit*
Best new one: Insidious 1-2 *demjumpscaresthough*
Best old one: The Thing *mostawesomeeffectsever*
Yeah, I guess that covers it