So I checked out our list; based off of what normally gets played and the bosses I haven't seen in a while, these are probably our down in the dumps bosses
Seeman
Staregineer
Bob the Builder
Wheatley
Servant Grunt
Sergeant Painus
Zerstorer
The Rock
Oogie Boogie
Joker
Pacman 2
Phoenix Wright
I've only seen these bosses at little times because someone forgot to actually choose their boss and was randomized to the following which was also followed by a "I didn't want this boss"complaint. But that's just my speculation, please feel free to add or disagree to the list!