if we’re not including djs then i’d say this one! https://www.mangago.zone/read-manga/can_t_take_my_eyes_off_you/
I've been into BL since the mid-early 2000s. it all seems like a blur to me now, but the first ones I remember are "Okane Ga Nai (No Money)"; "Junjo Romantica"; "Seitokaichou Ni Chuukoku (Hey, Class President)"; and "Koisuru Bokun (The Tyrant Falls In Love)"
i read it on webtoon canvas (it’s here too https://www.mangago.zone/read-manga/obnoxious_hero_kun/ ) i was like idk 12 yrs old probably younger it was my first time seeing gay sex i think the artist deleted everything and restarted cuz the upload date is different now
mine was the novel, grandmaster of demon cultivation aka “the untamed”