All of them - There is not a single good boss fight in this game, which is one of it's biggest issues. All of the bosses in this game are very underwhelming. They usually rely on having a whole bunch of goons to help them during the fight, and they're never difficult to take down.

Jingu - The Jingu fight is miserable. He runs away and shoots you the entire time while you have to fight off the soldiers that are defending him. If you knock out the soldiers but take too long to defeat Jingu himself, the soldiers will revive and you have to deal with them all over again. The soldiers are ridiculously tanky too. It's an obnoxious, all-around miserable fight and contender for worst fight in the entire series.

The Man in Black - The Man in Black fights are stupid because he'll block all of your attacks, prevent you from throwing him, and he does an enormous amount of damage with his knife boots during the second phase. But if you have a weapon in your inventory, you can use it to shred him to pieces with little resistance thanks to Heat actions. Easy to cheese, abysmal to fight normally.

Spider Guardian - I will never forgive or forget the Spider Guardian as long as I live. You can chalk this up to a skill issue, and y'know what, I won't argue that. It won't change how much I absolutely despised the fight. Having to carefully navigate your Morph Ball to the sections where you need to bomb to do damage to it and falling off of those slanted platforms constantly was god damn infuriating. I didn't die to it constantly, but I did have to redo the section where you need to jump onto the platforms and bomb the slots frequently because I kept failing to make it in time.

Dr. Cortex - I beat this boss first try completely on accident. It just wasn't difficult at all. I was confused when I won, lmao. Making the final boss a gimmick fight was pretty lame in all honesty.

Nidhogg - There's not much to really say about this fight other than it's just really easy and boring. It's twice as bad trying to fight it as Vergil as it clearly wasn't designed with Vergil's moveset in mind.

The Final Boss - I don't want to spoil it, and I can't really talk about it without spoiling it. It's just conceptually not at all interesting.

The Final Boss - Same reasons as DOOM II. I get what they were going for with this, I'm still not a fan.

The Imprisoned - It's been more then ten years since I last played Skyward Sword, so take what I say with a grain of salt. However, I remember the Imprisoned being a very frustrating fight that you had to go through multiple times. I remember it was annoying trying to break all its toes because every step it would send a shockwave that would damage you.

Bowser - The Bowser fight is just an extremely easy and underwhelming note to end the game on, especially considering how much harder it was to actually get to him than actually fighting him.

The Metroids - The Metroid fights are the one thing I feel like AM2R doesn't do as well as the 3DS version of Samus Returns. The Metroids' hurtboxes on their stomachs are really tiny, and it looks like your missiles should be hitting them, but more often than not, they don't, they just bounce off of them. It's really frustrating.