definately the child with a chainsaw

Dumuzid had the best one-liners, but I had to vote chainsaw.

Story-wise, easily Shadow of Vincent. 
Gameplay-wise, Thomas Mutton, I love how the game throws a wrench into your plans, with his gun.

There's nothing that says "Nightmares" to me more than a giant robo-baby with chainsaws for eyes.

I'm gonna go with baby with chainsaws for eyes, design wise. Gameplay wise Thomas Mutton because he was the only boss that I can remember actually doing anything. I found the bosses to be much easier and shorter than the stages that came before them.

the baby with chainsaws was my favorite based solely on its design.
 
None of the bosses were ever really a challenge though, especially since you could just nullify their special abilities with a press of the undo button.

I'm not going to pick the child with chainsaws because I spent so much time trying to beat him -_-. The Shadow of Vincent was the best design wise, Catherine is a close second though.

It was definitely Dumuzid, his ability to change block types randomly made that level a lot of fun.

Dumuzid fo sho

Dumuzid!

Child with a Chainsaw has just a Matter-Of-Fact name, it's great. Gonna go with Dumuzid though. None of the other bosses make Castlevania references or quote DMX songs.

Catherine. Something about having to work with Katherine while running from Catherine was a good time, no matter how long it took me to gold that damn stage. Close second would be Thomas Mutton cause he'd always shoot my fucking blocks and I had to learn to run around like a chicken with my head cut off so he wouldn't shoot the ones i needed. XD
 
@FancySoapsMan said:

the baby with chainsaws was my favorite based solely on its design.  None of the bosses were ever really a challenge though, especially since you could just nullify their special abilities with a press of the undo button.

Mutton.

From the standpoint of the boss design it was the child with a chainsaw, but I voted Dumuzid because the stage was really fun.

Dumuzid was a lot of fun to fight, but design wise I think the immoral beast takes the cake. That thing is terrifying.
 
Honorable mention for CHILD WITH A CHAINSAW. Now that I think about it, a lot of bosses in this game were fun to "fight". I pretty much liked all of them.

I voted Shadow of Vincent as, even though it didn't do as much as others during the battle, I loved the concept of it. I would have enjoyed it even more if it went all "Persona 4" and Vincent tried denying everything that his Shadow told him while climbing and, in the end, he'd turn actually accept his Shadow as a facet of himself that he tries to reject. It could have been the only nightmare creature that Vincent confronted directly and, instead of running away, the shadow would be destroyed by Vincent's realizations.

Combo of Dumuzid and Mutton, even though the camera angles during those stages drove me nuts. I felt like the climbing in those two seemed like a nice culmination of everything.

i love castlevania so as soon as dumuzid said "its a fine night for a curse" i was pretty sold on him being awesome