Artorias is simple if you don't let him buff, if you let him buff then you are in serious trouble. I have two bosses I have a huge problem fighting, Manus and Kalameet. I can handle Manus a little better than Kalameet but Kalameet usually takes me a few tries to kill, I am just horrible at fightning him.
"Villains!" I shrieked, "dissemble no more! I admit the deed! --tear up the planks! here, here! --It is the beating of his hideous heart!"
Hardest thing to do in my opinion is getting Kalameet's tail. In NG+, I think Manus is the most difficult to defeat. In new game, I think you should go for Smough first to make it easier in NG+. That is also difficult.
Pinwheel, simply because he's the first boss I fight so I can prepare to make my fire Murakumo :3
After that, Sanc gaurdian, Kalameet and Manus. They're all ridiculous! ESPECIALLY Kalameet. I actually wish there were more bosses like him though. Usually a boss will be super slow and powerful, or really fast but not too hard hitting. Kalameet is a hard hitter AND fast as hell. Love him.
I still remember my first time fighting him. Couldn't kill him without two summons, and they eventually died ;_;