I do feel the emphasis should lean Horror, keep in mind this will be the Top 100 Horror Films.
Based purely on the IMDb tags, here's how the on-the-edge films mentioned so far in here shake out:
Horror:
Tucker and Dale vs Evil
Shaun of the Dead
Kill List
Alien
The Thing
Psycho
Army of Darkness
Bubba Ho-Tep
Sleepy Hollow
G̶r̶e̶m̶l̶i̶n̶s̶
Fido
The Fly
Invasion of the Body Snatchers
Not Horror:
A̶l̶i̶e̶n̶s̶
T̶h̶e̶ ̶S̶i̶x̶t̶h̶ ̶S̶e̶n̶s̶e̶
G̶h̶o̶s̶t̶b̶u̶s̶t̶e̶r̶s̶
J̶a̶w̶s̶
T̶h̶e̶ ̶S̶i̶l̶e̶n̶c̶e̶ ̶o̶f̶ ̶t̶h̶e̶ ̶L̶a̶m̶b̶s̶
The IMDb tags sort a good bit of it out.
To use the example of
Ghostbusters, consider how the elements work inside the story. Are the Ghosts handled inside the story as a horror element or as comedy? Think of the big cute Marshmallow romping through the city. That sends my mind to Comedy. There *are* Horror Comedies, yet to be a Horror Comedy the movie has to feature Horror elements that compete directly with the comedy.
Of the ones tagged by IMDb, I don't count Gremlins as horror. There are several others on the borderline as well.
Horror genre is very broad based. In the last several decades it has been a lot of slasher gore stuff...but before the 70s that wasn't usually the norm. I think all of these classics horrors are still horror:
Psycho (1960)
The Exorcist (1973)
The Shining (1980)
Rosemary's Baby (1968)
Frankenstein (1931)
Carnival of Souls
Dracula (1931)
The Old Dark House (1932)
Poltergeist (1982)
The Haunting (1963)
The Omen (1976)
I think we need to allow as many MoFos as possible to participate by keeping the horror genre broad...If this was just all slasher gore stuff I won't be able to send a list.
These are all undoubtedly Horror.