Well MM in my opinion has the best boss fights of any Zelda game ever, but within the game I'd have to say Twinmold. It's the biggest boss you fight in any Zelda game, that's been to some debate before, but when you think about it... Twinmold is freaking huge. What I love about all the boss fights in MM is that unlike every other game, there's no specific set up way to beat them. It's not like "Throw bombs at them to reveal their eye, shoot the eye, hit them a few times, do it again, do it again." I hate that, it's so set up, in MM it's like "Hey here's a giant green warrior thing who's incredibly drunk and has a giant freaking sword, kill him." And that's it. Or with Twinmold, "Hey here's two insanely giant flying insect things, kill them." And there's so many different ways to! It's not set up! That's why MM is the best Zelda game. Well a huge reason.