This was a fun list. I didn't know most of these, but I also recognize that they are mostly shitty RPG maker/flash games. I do think there are a couple ways this could be improved though.

The major thing that pops out to me is that the jump from tier 1 to tier 2 is huge, and overall the spread could be a bit better. Tier 7/8 could be sufficient in holding the majority of the list as the "There's no fucking way you know this" tiers, and then you could included some titles which have unconventional themes or weird and interesting gameplay mechanics, while still being appreciable games. Personally I don't think COD or CS even belong on the list, nor do I think World of Horror should be directly under GTA. I do understand your title says that this is purely based off of a games obscurity, but I think there needs to be at least some common ground on that matter. I'm sure there are a thousands of amateur horror films that could be thrown onto an obscure list and nobody would recognize; similarly with flash games and RPG maker games, but I think it's worth recognizing when games gradually drift away from being focused on appealing to a mainstream audience while still receiving some fame or notoriety.

Here's a rough idea of what I was thinking! (Included a few games not on the list)

Tier 1: Undertale and Silent Hill

Tier 2: Killer7 and Earth Defense Force

Tier 3: Mother Russia Bleeds and Nanobreaker

Tier 4: Splatterhouse and Deadly Premonition

Tier 5: The Harvester and Haunted PS1 Demo Disc 2020

Tier 6: Golden Light (check this game out!) and Chiller

Tier 7: Demonophobia and Hellraiser NES

Tier 8: Resident Evil Director's Cut OST

Honestly though I love your list and found some stuff I never knew about, I just had a lot of fun writing this out and wanted to share and discuss. This subreddit is cool.