Twinrova!
Trinexx was my favorite boss in A Link to the Past, and it was so cool to see that fire/ice idea make a return in Ocarina of Time. I thought the idea of the twins taking form as one character was very cool, and and absolutely loved how Link blushed when she winked at him, heh heh heh. Using the mirror shield was also a really nice touch, and running around the platform to avoid damage was cool, too. As soon as I downed Twinrova, I'd leap across the platform and come smashing down on her with the Megaton Hammer! It was so much fun.
Plus, the bit at the end where they are quarreling even in death was pretty funny and added some needed levity to the situation since, you know, Nabooru had been brainwashed and you had to fight her and the story was pretty deep at that point.