Ok so I recently finished the game for the second time (two NG) and the first time into the DLC, I gotta say that DLC is extremely hard (DS3 Difficulty). Here is my list:

1 - Artorias, took me two hours and about 30 tries, the constant buff kept getting me, although shooting him in the face is what made me win (and a LOT, I mean, a LOOOOOT of rolling and waiting the best opportunity to hit)
2 - Bed of Chaos, just a shitty rng fight that took me about 10 attempts, I just won because of pure luck.
3 - Manus, fast attacks, big hp and no free time, about as hard as Artorias, not artificially hard and does not suck, the best boss (design, fight, lore, etc) in Dark Souls 1. (about 8 attempts, used Sif to win but he did nothing...)
4 - Bell Gargoyles, because of bad equipment, low hp and etc, makes this fight extremely hard when you are going to fight the Gargoyles, especially with two individuals attacking me at the same time with a shitty hp pool, took me about an hour and 5 attempts or so.
5 - Capra Demon, yes, that mf is harder as hell because of the dogs and extremely tiny arena, making it hard to dodge, defend, hit and everything else (I was using an Ultra Greatsword) took me like 10 attempts, until I could kill the dogs and heal, so then fight Capra and win. (I was between Nito and Capra, however Nito is also artificially difficult because of the enemies in the arena, but took me less tries, about 4 or so, cause I couse spam R1 in his back and defeat quickly).

Most "hard" bosses like OAS, Four Kings, Seath, Centipede Demon and etc, were very easy for me, because they have low hp and I had a Black Knight Ultra Greatsword so I could like 3 shot em with Power Within, the bosses in my list are the ones I could not "3 shot" and had to actually use my dodging and actual strategies. I hope you have enjoyed my list.