Pretty obvious poll.
Not your favorite Boss, Most enjoyable battle, but the most Difficult boss to beat, preferably the one who you lost to a few a times, and then you reliazed your last save was hours ago, etc...
Furthermore…
I’m not referring to NG+ (New game + Except for in one battle) , and I’m not referring to the Guy’s who bothered leveleing them up to lv.40 right before Yakra in the Cathedral.
Or in other words, "Most difficult battle, At the excepted level for that Boss"

Mine was definitely Miguel and Lavos at the Ocean palace (Took a Newgame+)…