There are many interesting, well-detailed bosses in the game. Malistaire, Jade Oni, Meowiarty, Queen Calypso, Sylster, Lyon, etc. But out of every single boss in the game, I have to say my favorite is the final boss of Celestia, Mithraya. Her details and her physical attack are just amazing. I would love to have a pet version of her or even a spell for fire. Thats just my favorite boss, what about yours?
I find any member of the Shadow-Web or Morgantine minion to be a delightful boss ( sorry Headmaster Ambrose but my heart belongs to Morganthe and her minions )
out of EVERY? wow thats a tough one. i guess i would have to say... the sons of ymir. probably cause you got to know them as the kings of wintertusk and then after all the help they gave, (if you call it help) and then they try to kill you. plus they are the only boss that i like when they cheat. i mean, they play tag while trying to kill you. thats kinda fun.
Personal was always a mix: Lord Nightshade/ Agony Wraith / Oyotomi the Defiler. Lovely to look at, every last one. But if you made me pick, it would be Agony Wraith every time.