Koloktos is easily my favorite boss in Skyward Sword. I like the whip and I think it's a little underused in the game so this lets the item shine for a moment. It's packed with action and it's not interrupted by unnecessary cutscenes or waiting for the boss to make a random move to expose his weakness (at least if you know what you're doing). The battle also brings back one of my favorite mechanics from the Wind Waker - picking up enemy weapons. It's kind of weird that they designed something like that merely for one boss fight. The boss is also graphically the most interesting looking, it's got the best boss music in the game (although it's shared with Moldarach) and even the name Koloktos sounds pretty cool when you pronounce it.
Either Ghirahim V3, Koloktos or Demise. I liked the challenge and how they actually felt like fights. The rest of the bosses felt pretty lame in comparison being the generic "3-hits and dead" brand of boss. Thought, the battles with The Imprisoned with Groose helping you out was pretty cool. I'm surprised how seamless and fun a co-op boss battle in a Zelda game was.
Hard to decide. There are several good bosses. The later bosses were veery nice. I liked Demise. I also liked the fights leading up to it. It seemed really epic. Kolocktos was good. The later Imprisoned fights are good.
Bilocyte seemed more like a miniboss than a big boss.
I'm leaning more on Ghirahim's side since his battles were just too fun, and difficulty definitely exists. He doesn't have the usual Zelda "weakspot" of shooting an eye or some obvious part of the body.
Whenever I go to fight something at the thunder dragon I keep selecting Demise, so I gotta go with him. Ghirahim would be my choice if he wasn't so darn easy. Once you figure him out he's one of the easiest bosses in the game.
Demise however remains the most difficult for me to predict even as you understand how his mechanics work.
I liked Tentalus because of the sheer awesomeness of the sound effect that plays when he is vulnerable, but I had to go with Koloktos. Grabbing your foe's swords and hitting it with it is a fantastic concept and I hope it gets used more in future Zelda games.
Currently, my favorite would be Scaldera. It has really been the only boss who I had some fun fighting.
Koloktos was kinda fun though. But I guess I didn't enjoy it as much at first because half of the time I had no idea what to do. <_< It took me awhile to realize that I also had to slash its legs off in addition of its arms.
I loved all the fights with Ghirahim, but round 3 was most definitely my favorite.
It was similar to the other two fights, but still unique and challenging in a different way. I felt so epic jumping down from the platform with a final blow. When he made that BIG ASS zweihander appear near the end of the fight, I nearly wet myself.
Others I enjoyed fighting were Koloktos (though not initially) and Scaldera. It took me forever to figure out you had to throw a bomb in Scaldera's mouth. I felt so stupid after, because it was an obvious reference to King Dodongo from Oot!
As soon as I saw him sucking air I knew, though at first I didn't know I needed to follow him down when he rolled to the bottom. I liked those fireballs he spat. It was a good boss. Even when he's stunned, he can still move that eye around, which was a first I think.