There's a lot, but I think the one I'm most surprised I haven't played yet is Omori.
I've played all the ancestor horror pixel RPGs: Ib, Mad Father, The Witch's House, Yume Nikki, OFF (and Undertale and Lisa to an extent, but they're not really horror). I've also watched playthroughs of Misao, Corpse Party, The Crooked Man, Hello hell...o?, and Ao Oni. With the extensive library I've had with horror pixel RPGs, you'd think I'd have played Omori LMAO. I'm honestly dying to play it, but I'm just waiting for it to go on sale in Steam, so I'm stuck in limbo. o(-(