Morpha, because there's always the chance that you miss pulling his core thingy out of him. Though it's not hard to avoid him grabbing you, and there's a chance his second tentacle gets in your way, and Gyorgh: Fail jumping onto the platform and you'll get eaten.
i had the most trouble with phantom ganon, it took me a bit to get the timing down and my total health was still kinda low. bongo bongo really isn't that bad you just need to be aggresive. with morpha stay in the corner and be patient, the first time i tried i thought you stayed out on the platforms and died in about 3 min -_-
I despise Bongo Bongo and Morpha the most. *SORT OF SPOILER*Bongo Bongo just plain scared the living daylights out of me the first time I played, plus each hand is like a boss within itself that you have to destroy. Morpha is annoying with the way that.. um.. he? She...? grabs you and flings you about. Plus, sometimes I would grab that annoying little bouncy thing with my Longshot, and it wouldn't quite make it onto the platform, so I couldn't slash it with my sword. Very irritating indeed.
For me it's either Invisible Penal Monster(Bongo-Bongo) or the Jellyfish in a puddle(Morpha). I call them that out of spite. And yes I know Morpha is an amoeba and not a jellyfish. But back when I played OoT for the first time I mistook Morpha for a jellyfish. So now that's what I'm calling it. Out of spite.
Why does everybody hate Bongo-Bongo so much? He was easy and one of my favorite bosses in OoT, actually in every game his similarities are in too! Bongo-Bongo(OoT) The mechanical thing in the tower of gods(WW) and Mazzal(MC) They all share a common weakness too, arrows to the hands!
I don't know if this counts for anything, but I thought that in Spirit Tracks when your playing the song with the Lokomo of the sand region was one of the hardest things I've done in any Zelda game up to date. (A second would be the statue thing in Twilight Princess outside of the Temple of Time.