Wow. The title really threw me off:
"Favorite MM boss (excluding Majora's Mask)" I was thinking: Favorite boss in Majora's Mask except for the bosses in Majora's Mask...?!
I voted for Odolwa, mainly because he has the most unique strategy and array of attacks. I find him the most fleshed out and interesting to fight out of all of them, as well as being vulnerable to a large range of attacks later on (giving you some reply value as well as interesting strategies). A lot of the other bosses only have one or two ways to fight them, especially Twinmold.