I thought it was Blizzeta. Blizzeta was so much fun to fight. You just skated on the ice trying to avoid her falling on top of you. Then, you could break her little chamber and just go from there to finish her off.
I loved Zant. He was challenging. It wasn't easy to figure out what you had to do on the first try. It was a fun battle and I enjoyed going back to a few of my favorite temples to revisit them in a new way.