One per game, if that isn't too inconvenient.

Does hard include bullshit (bed of chaos)?

For me it was Kalameet, from the dlc of the original Dark Souls.

There really isn't a specific hardest boss per game. It's very subjective. But the ones I've heard the most complains about throughout the years, mainly in this wiki, are:

DeS: Old King Allant

DSI: Ornstein and Smough

DSII: Blue Smelter Demon

DSIII: Nameless King

BB: Orphan of Kos

ER: Malenia

Sekiro: Don't remember

From made other games, but I guess these are the only ones where people like complaining about bosses a lot.

Sekiro would prolly be either Demon of Hate, Prime Isshin or Owl Daddy, leaning Deamon of Hate

Also not putting Fume Knight as the hardest for 2 is wild (i didn't find him too hard but the stats speak for themselves)

Those are not the bosses I find the hardest, only a few of them I do. Blue Smelter is one of the bosses I remember multiple people posting that they're just skipping for being too hard.

In my opinion Burnt Ivory King is the hardest boss in DSII.

Honestly that's prolly more due to a combo of A Previous Boss, BUT BLUE and the runback than the actual boss itself, in which case Lud and Zallen would 10000000% be up there for living in Horsefuck Valley

DS- Manus

DS2- Dark Lurker

DS3- Nameless King

Sekiro- Old man who’s name escapes me

Bloodborne- Orphan

Elden Ring- Goddess Of Thot

For me

Dark Souls - Manus i think, been a while since i played

Dark Souls II - Sir Alonne

Dark Souls III - Midir

Bloodborne - Laurence

Sekiro - Inner Father

Elden Ring - Malenia

Demon's Souls - Flamelurker

Dark Souls - Knight Artorias (Manus and O&S are pussies!)

Dark Souls 2 - Sir Alonne

Bloodborne - Orphan of Kos

Dark Souls 3 - Sister Friede

Sekiro - Inner Father (either Owl (Father) or Demon of Hatred if only counting base game)

Elden Ring - Malenia, Blade of Miquella

Armored Core 6 - The True Final Boss (need to reach NG++ to fight them; not spoiling it)