I have problems deciding which Bloodborne-bosses were the best

DJ Octavio in Splatoon 1 for moi.

He's already a fun boss for the first phase of the fight, but that second phase, when the Squid Sisters music kicks in and you blast and dodge your way through a hail of missiles and kick his arse to that glorious tune, holy crap it's borderline orgasmic.

The first Baldur fight from god of war. It was just so cool and surprising and crazy and intense and satisfying.
The balder fights in bayonetta 2 are great, particularly the one where the summon are fighting as well in the background.
Mollusque from Mario odyssey, i love the way this fight uses the entire map, chasing him down was just a lot of fun.
Doc ock in spider man.

Persona 5 boss fight against Shido simply because of the fighting music

Wonder if Sekiro will be heavily mentioned in this list soon. Quite a few soulsborne mentions here.

I'll wait for Sekiro to decide the finale, but God of War's Baldur was one of the best. What about those machines from Horizon though, Redmaw/Thunderjaw was also a great boss battle not really hard but still cool to think I'm fighting a Mech-T Rex. And there's Bloodborne's Maria and then Orphan of Kos, only reason I added him was because of him, DS3 was a cakewalk, I forget but I'm sure I've played more, these four are the only one's on top of my head.

Considering that Dark Souls literally invented the action genre and that there hasn't been a good action game in the last 8 years that wasn't a Soulslike I wouldn't be surprised. From Software are the only ones who know how to make cathartic bosses because it says on the back of the box "Prepare to Die". 

Why not name some? I beat the standard game(no dlc) and the only memorable fight was the lightning dog.

Medeus, from Tokyo Mirage Sessions: #FE

Thinking of different ones most you guys have already mentioned.

The first time you fight a Dragon in DA:I.
The hardest cauldron in Horizon (it has a T-rex and Tiger there).