The End

You can tackle this guy in any way you want to, and I love that since you can use pretty much all of the game's mechanics against him, plus there's some fun and cheeky ways to beat him too

Vergil 3

Fun to fight with a nice variety of moves, great aesthetics and music, and hard as balls, especially on DMD. Also it's Vergil.

Lingering Will

Same reasons as Vergil, but a little worse since he's not a 40 year old weeb like Vergil. Still a kickass fight tho.

Masato Aizawa

Brutal multi-stage fight that pushes you to your limit

Grigori

ABANDON ALL DELUSIONS OF CONTROL

Henry Cooldown

Really damn good fight that pushes you to your limits and tests all your combat abilities. Also has the best boss theme ever.

Lechku and Nechku

Fun double boss with an interesting gimmick, great presentation, and the best boss theme in the game

Burnt Ivory King

Fun multi-stage boss with amazing presentation and great design

Purity of Odio

Fantastic climax to the game with great story significance. Only issue is the boss theme isn't as good as Megalomania but what is.

Shadowlord

Good rival fight with great story significance and the best song in the game. The improved combat in Replicant gives that version the edge.