Ornstein & Smough for me. Totally ruined the whole thing. I haven't touched the game since.
It gets much worse on NG+. I did co op with a guy (helping me) he had hgh vit and havels armour, he got 1 hit killed by smaughs hammer rush.
On your first playthrough just use soliare nd maybe summon someone if there is sign. My advise, kill smough first, ornstein is easier imo. It also annoyed me in dark souls after finding out enemies attack through walls. Not jsut ghosts, but thinks like titanite demons. I dont think I will play it again tbh, NG+ makes it just too annoying for me, in black knight armour +5 channeler stabbed me and insta killed me. Yet they take 3 hits from my black knight halbard +5.
I would rather an option to lock difficulty, but it started off as fun, with nnoying things. But NG+ took away more of the fun, and made it more annoying.