Oh, the DLC is definitely another level!
From the vanilla game, I'd say the gargoyles were definitely a real test for me. I was a noob back then, I didn't know the mechanics and I didn't understand some concepts. I had to understand several things, like the weapon upgrades, to beat them. Right after that, I wouldn't say the bosses weren't that hard for me. I remember that my friend freaked out when it took me 4 tries to beat O&S, and a couple of them to beat the Capra demon
But the hardest of all of the soulsborne games, I'd say is Malenia 100%. It literally was the first time that I got stuck like a couple of days in a boss.