Which From Software (Souls/Bloodborne) Game is hardest?

[Runa216]

Well, I'm swelling and buzzing with pride right now because I just conquered Bloodborne. 

I didn't just beat it, I conquered it. I got all the trophies, and absolutely decimated the final bosses (Seriously, that Pthumerian Queen went down like a bitch!) and have nothing left to do! 

so now that I've beaten Bloodborne - the latest game from From software, the makers of Demon's Souls and Dark Souls - I have to ask, what do you all think is the hardest game made by then. I only ask because I played but didn't beat Demon's Souls, I bought but haven't played Dark Souls, and am waiting on getting Demon's Souls II: Scholar of the First Sin on PS4. I've heard different people saying different games were harder, some saying they were easier, and I was wondering how this one compares so I can tell if I'm worthy. 

I've heard Bloodborne is the hardest, but also the easiest due to its emphasis on combat and attack, but I also imagine realistically that people only find it easier because they've been tampered on Demon's Souls and Dark Souls. 

So what do you think is the hardest, and please give a reason WHY: 

1 - Demon's Souls

2 - Dark Souls

3 - Dark Souls II

4 - Bloodborne

And for some light youtubing: https://www.youtube.com/watch?v=gLsc-U8b4E8