There are several factors to consider
1) usually anyone’s 1st Fromsoft game feels the hardest because they are not used to such games
2) Sekiro maybe objectively the hardest game because there is no way to side-step the need to git good or to really improve your skills. Also it has a high number of truly challenging bosses
3) Bloodborne is a close 2nd with hard DLC bosses
4) Rest all can be bunched together. From these games, only certain bosses stand our as being hard such as Malenia, Darkeater Midir, Sister Friede, Maliketh, etc. In these games, magic builds can be OP and break the game’s difficulty