I voted for Titanite demons. The handful at the bottom of Sens, and really the respawning one in Lost Izalith seem to consistently trouble me. For the ones in Sens it's usually my own over confidence. The bugger in Izalith just seems to have my number.
On my first playthrough it was easily the black knights, though. --- "Life is like a box of chocolates. Diabetics can't enjoy either."