I've only fought Black Knight (twice), Specter Knight, King Knight, Plague Knight, Treasure Knight and Mole Knight so far. Of those:
- Black Knight is the most satisfying all-around fight -- dodging his drop attack, and then pogoing off him in return!
- Specter Knight has the coolest projectile animation, throwing the whirling scythe. Reminds me of Trish's one move in Marvel vs. Capcom 3.
- Plague Knight is probably my favorite, in terms of his concept (alchemy). I also liked his manner of movement, leaping everywhere like a crazy person.
- King Knight wasn't particularly notable, but I do like his look.
- Treasure Knight looked like the main character from BioShock 2, but it's not like those games invented diving suits. I really dig the vertical and horizontal hookshot he used.
- Mole Knight moved fast but was still a bit easy. Great variety of attacks, though.
All of the bosses have been stellar, really. Looking forward to the rest!
ALSO: Baz and Reize (map mini-bosses) were great. So was the boss of the Hall of Champions.