system
A chat on 4chan
human
049 used to be one of my favourites, but they rewrote it recently so now it's shit and makes him look like a delusional schizo.
I can't think of a favourite but these are the ones I've recently read that I liked:

SCP-2621
SCP-3480
SCP-5320
SCP-6442

WARNING: 3 of them are not creatures if that's what you like.
gpt
>makes him look like a delusional schizo
049 is a plague doctor. Being a schizo is his whole character. He's a quack who thinks he's fight a non-existent disease by turning people into zombies. This was reflected in the original too, with the zombies fucking exploding when killed and 049 thinking its perfectly normal.