Played Bloodborne about 4 or 5 times and am currently juggling a NG+ and new NG0 wretch save on Elden Ring, and I think it's Elden Ring.
But I look at it from the perspective of what it is like to fight these bosses if you aren't overlevelled and using helpers. I think tbh it's the way the fights should be looked at. Even though Elden Ring does make it easier to be overlevelled, and gives you more helpers. But you can still summon real and NPC summons in BB and get OP from chalice dungeons, so it's not like it's impossible there.
Combat encounter design wise, there's a level of ambiguity in how some of the enemies approach you in Elden Ring that demands you either go the OP route, or study the fuck out of these encounters. They are just way more technical fights now, with way more ways to approach it. Yes there are busted builds, but the effectiveness of some of those builds will be lessened overtime by patches anyway (software not Patches the dude).
It's the first one of these games where I've felt myself thinking about my positioning in a fight, the recovery on my high commit attacks and dodge rolls. I never had to square off to a boss in BB like I have to in Elden Ring. Something like Maria in Bloodborne would seem like a really easy and trivial encounter if designed in a similar fashion in Elden Ring, next to the rest of the cast (by which I mean a slower version of that fight, cause you're faster in BB)
Tbh, I don't think there's much that's monstrously tough in BB. Gascoigne is a fearsome skill check, but from that point on it's not an especially difficult game I think. I think Bloodstarved Beast is a bit fucked up. Rom has an annoying gimmick which is called 'wasting your time with a boring fight'. It gets well hard again for the last fight and the DLC fights. But I think even the very first 'main' boss in Elden Ring is already at an Ludwig level of difficulty if you fight him at a non OP level, where you're dealing a reasonable amount of damage and him to you. If you're just nuking the boss and he's doing nothing to you then I just think you're too high level for the intended encounter design anyway, that's how I look at it