Not OP, but I have very strong opinions on this, so Imma chime in. I'm very much on the "older Souls is better than newer Souls" side of the argument, and I think Demon's Souls has the best bosses in the entire series, hands down.

Bosses used to be unique, and used to provide interesting challenges to the player, from their arenas to their movesets to their "gimmicks". Every boss in DeS is different, strikingly so, and they all add to their settings and make use of mechanics in interesting ways.

Nowadays, all boss arenas are just enormous empty spaces, with nothing to impede the player, and all bosses are just rhythm games with slightly different rhythms. It's just boring.

Demon's Souls bosses are interesting and innovative. People think that "most of the DeS bosses are gimmick bosses, with only a few real bosses", but what they don't understand is that every boss in DeS is a gimmick boss, and that's what makes them interesting. False King Allant's gimmick is that you have to learn his moveset and master the combat system, as opposed to mastering stealth in the Old Hero's fight, or solving the puzzle of how best for your build to damage the boss in the Adjudicator fight.

Demon's Souls bosses were like their levels - experiences to be explored, to be conquered through your ingenuity and use of the tools the game provides, not just "are you good enough at DDR and min-maxing your build?" This follows the general (unfortunate, in my opinion) trend of Souls games coming to be more focused on combat instead of overcoming, because while they sound identical they're actually two very different things. Level design too has completely fallen by the wayside as combat has become the sole focus of the games' challenges for the player.

Everyone loves Artorias (and I do too), but he was the beginning of the end for the old, slow, deliberate adventure style of Fromsoft game that I fell in love with the series for in the first place, and while I love the newer games too, there's very little out there like Dark Souls 1, and there's nothing out there like Demon's Souls.