Honestly I liked most of the bosses..

[The Dopplegamer]

At least the humanoid ones (plus Romina). Then again, I love the parry play style. Rellana, Mesmer, Midra, even the final boss can all be easily parried, especially with golden parry. However, I am curious to see how I feel when I attempt my next playthrough with a spear/faith build. the tight punish windows don't seem conducive to a casting play style...

[The Dopplegamer]

Plus it was cool to get an actual Knight Fight, a From Soft boss staple. The base game was really lacking in those in the boss department (Melania being the closest thing)