Hmm, let's see.
***SPOILERS! INCLUDES ALL KH3D BOSSES!***
(please note that I played the Standard mode)
Hockomonkey
(Sora/Riku) - Pretty easy. Although it was the first boss, even the rematch version wasn't that hard.
Wargoyle (if I got that right)
(Riku) - This one was actuall a toss-up. I beat it on the first try, but it took some genuine effort.
(Sora) - Oh, good grief. This thing was a PAIN! It took me several tries to get it. GRR!
Char/Chill-Lobsters
(Riku) - Firecrab got burned! The upside-down thing wasn't that tough, and the boss was easy to figure out.
(Sora) - The only real difference here was a colder entree and a cool end-battle scene.
Rinzler/Commantis
(Riku) - The mantis took me a couple of tries to get, but once I got the pattern it was game over.
(Sora) - It was a pretty cool battle, though I think I kind of cheated. Whenever Rinzler flipped the screen, I just mashed Y to guard. Not too hard, but somewhat challenging.
Holey Moley/Pete
(Riku) - GAH-GRR-GUFF-GRGHGHGHGH!!! This is the most annoyin boss in all of the KH universe! The stupid Hyenas in KHII were less annoying than this!
(Sora) - Meh. It had a very small challenge to it, but the scenes were fun.
Chernabog/Spellican
(Riku) - Cool! A drop boss! This was one of my favorite boss fights, actually. Shame you can't retry it.
(Sora) - Boy, this stupid bird was tough. And those brooms! Those electric-shooting, pain in the hinie, brooms! It took me a few tries, but I got it. This is an ideal boss, right here.
Xemnas
Not bad at all. I actually had to go back and level up a bit before I could beat him, but he was a pretty good fight.
Ansem - SoD
(Form 1) - Classic. A good, tough fight and solid battle overall.
(Form 2) - This one took me a few tries. I do kind of wish this one had a replay; it was fun.
ABCN
Boy, at first - hardest boss of the game! After five minutes - piece of cake. It basically repeats a pattern of attacks and once you know those, you can easily kick its butt.
YMX
This is my favorite boss of the game. Awesome battle, awesome plot point, awesome background music. Glad you can re-fight him.
Final Boss (to prevent spoiling)
Very tough, and well done. A good fight to end the game on. It was as tough as it should be and not too easy.
Julius (I doubt this name alone will spoil you)
A very obscure, but cool referance. It took me like 30 tries, but I got the ULTIMA KEYBLADE! Although Unbound's better, in my opinion. Very hard secret boss.
Anyone else? No? Well, there's my opinions.