BackFire
Blood. It's nature's lube
Nightmare on Elmstreet - The first one. It was the only one made out of passion for the idea rather then simple money, and it shows. all the sequels are pretty lousy in comparison.
Friday the 13th - Jason takes Manhatton. The only entertaining one in teh whole series, sure it's stupid, but they all were, at least this one was stupid to the point of being entertaining.
TCM - The original, for obvious, undeniable reasons. Such as is overwhelming influence on all horror films made after it. And it's stark, realistic nature that shocked audiences when it was first released. No horror film of the past 35 years has been more influential.
Trilogy of the Dead- This is the hardest one, because they are all so great. I think they are all equally strong for different reasons. Night of the living dead because it was so damn original when it came out. Dawn of the Dead because of it's strong and interesting message about society. And Day of the Dead because of the gore and creating new aspects for the zombie genre. All in all, I guess I'd say Dawn of the Dead is the best.
Halloween - I like the first one the best, though I'm not a huge fan of the series and don't particularly care for any of them too much.