Personally, I would say someone like Jason X, my reasons are as per below:
Leprechaun: Even though these can be pretty freaky, at the end of the day if you did come across a Leprechaun you are probably going to be bigger than it and be able to kick it's butt.
Wishmaster: Although this guy is pretty scary too, the fact that you know in reality he could never come about makes him less scary than Jason X.
Chucky: Again, as with the Leprechaun, he is pretty small and as long as you keep your wits about you, there is no reason why you couldn't just put him in the washing machine and set it to spin.
Freddy: Now, with Freddy, he came a close second to Jason X. I mean, definitely has the scare factor perfected to a tee, but I think in reality if a guy had those gloves, they would be really hard to use and all you would need is a baseball bat and you would probably have him. I know he comes to you in your dreams and all that, but that isn't going to happen in real life.
Alien: Basically just not real so no need to fear them.
So, that brings me to Jason X. Now, I know he is not real too as with all the others BUT at the end of the day, there is every chance that a massive dude could don a mask and a machete and go round killing people. I mean, if someone decided to "impersonate" Jason, it could be done, and I think thats what makes him the scariest, knowing that in the back of your mind while you are watching the movie, there could really be a guy out there waiting with a machete to have some fun!
Leprechaun: Even though these can be pretty freaky, at the end of the day if you did come across a Leprechaun you are probably going to be bigger than it and be able to kick it's butt.
Wishmaster: Although this guy is pretty scary too, the fact that you know in reality he could never come about makes him less scary than Jason X.
Chucky: Again, as with the Leprechaun, he is pretty small and as long as you keep your wits about you, there is no reason why you couldn't just put him in the washing machine and set it to spin.
Freddy: Now, with Freddy, he came a close second to Jason X. I mean, definitely has the scare factor perfected to a tee, but I think in reality if a guy had those gloves, they would be really hard to use and all you would need is a baseball bat and you would probably have him. I know he comes to you in your dreams and all that, but that isn't going to happen in real life.
Alien: Basically just not real so no need to fear them.
So, that brings me to Jason X. Now, I know he is not real too as with all the others BUT at the end of the day, there is every chance that a massive dude could don a mask and a machete and go round killing people. I mean, if someone decided to "impersonate" Jason, it could be done, and I think thats what makes him the scariest, knowing that in the back of your mind while you are watching the movie, there could really be a guy out there waiting with a machete to have some fun!