This is such a tough choice! I actually had to go through all of the ZD Enemies lists and look at pictures of all the enemies before I was able to come up with my decision... there are so many good ones!
In the end, I'm going to have to say that my favorite enemy is the Lizalfos in Skyward Sword. In previous games, my fighting style was usually rush in and attack, not paying attention to my health meter, so my first encounter with a Lizalfos in the Earth Temple really challenged me. In a way, it was the first time I really developed a legit fighting style. I started using backflips and side jumps more as well as my shield. The first Lizalfos killed me once and got me down to one heart a second time, but by the end of the game, I was able to take on two and barely take any damage. It gave me a great sense of accomplishment, so that's why it made my number one favorite enemy!
Now if we're talking bosses, I have to say it's a toss-up between Ghirahim and Zant, both of whom remind me of each other in a way, since they were both sort of "secondary" enemies when held up against Demise and Ganondorf respectively. They both have such a wide range of personalities that they almost seem more dangerous. With Demise and Ganondorf, you knew they were always angry so you were always on your guard. Both Ghirahim and Zant were so gentle in some ways that they seemed more dangerous because you never knew when they were going to snap. They were both also challenging; Zant because he revisited all the previous boss battles and put spins on them and Ghirahim because instead of button-mashing, the battle was slower-paced and more of a battle of wits than anything, which I loved!
And that's... my answer! ^^ :yes: