Inspired by the 'which boss do you like best' thread. I felt it lacked an option to say which fight you least enjoyed.
For me, that'd be Sindragosa ("your pathetic magic will BEEETRAAAAAAY you *hack cough*"). The voice acting is atrocious there and it's just way overused. Rotface suffers a bit from the same problem ("Good news everyone, I fixed the poison slime pipes!").