I think it has to be the pinwheel demon. I remember back when i played this game for the first time, I was a knight, and ended up killing him with a lightening broadsword or long sword, in like 5 consecutive hits. When he was dying, I thought he had to be queing up for phase 2 of the fight. 2nd place goes to the butterfly. He's another joke pushover.
Pinwheel is tuned to the strength of the Bell Gargoyles, as he's accessible as a boss pretty much from the beginning. He will murder you if the fight lasts too long with his clone-magic-spam, so in some ways he's a gear-check on your ability to do and/or take damage. The thing is the catacombs tend to be such a pita without divine weapons that no one bothers to go to there early, even if beating him gives the rite of kindling. Unless you want early gravelord access the skeletons just aren't worth the trouble.
That said, you can actually make Pinwheel into a difficult encounter by wearing heavy armor and wielding a slow weapon like a club. He is actually agile enough to jump/teleport away from you efficiently, and once he has enough clones the magic spam is truly dreadful.
Naturally summoning Leroy trivializes him, similar to summoning Solaire and/or Lautrec on the gargoyles.
Bane even prayed for Dick when he became Batman. -- Frostbite_Zero