"The Window next door" - by Junji Ito.
I have read Uzumaki, the enigma of Amigara Fault, and other short stuff by Junji Ito, but the window next door is the scariest to me, and the weird part is that...you don't even know if the creature in this comic is actually evil or just some deformed being.
In other Junji Ito comics, you don't know what the supernatural thing is, but at least you can tell is evil or how it will kill you, but you have zero answers from the window next door. And it doesn't help that to me that creature has one of his most creepy designs.