I also played demons souls first so it was relatively the hardest but none of them are actually hard and in actuality demons souls is the easiest by a large margin
I really wish they would've expanded upon that PVP boss from the Tower of Latria in Demon's Souls. That was an exceptionally clever boss fight - pulling another player from their game and making them literally the final boss of that area.
They did it in other games too. Looking Glass Knight in 2 and Halflight in 3. ---