Hmm...tough call since I don't just leave out certain games but also some of the elements within the games themselves. Ah well, I'll try to make this easy on me.
Castlevania: Lament of Innocence -Mathias is not Dracula. Instead he becomes the future King of Hungary-
Castlevania: Legends -Japanese story over US version-
Castlevania III: Dracula's Curse -Japanese story over US version-
Castlevania: The Adventure -Not adventure Rebirth-
Castlevania II: Belmont's Revenge
Castlevania Chronicles -I don't recognize the feminine pink-haired Simon as canon, but his original X68000 counterpart instead-
Castlevania II: Simon's Quest REDACTED -Definitely by far a good attempt to fix the issues-
Super Castlevania IV -I count this as a sequel to Simon's Quest due to the similarities with the Dracula tombstone. That, and Slogra and Gaibon make their first appearance in this game-
Castlevania: Harmony of Dissonance
Castlevania: Rondo of Blood
Castlevania: Symphony of the Night
Castlevania: Circle of the Moon -In my headcanon Nathan graves is the son of Richter's and Anett's daughter whom married into the Graves family-
Castlevania: Order of Ecclesia
Castlevania 64/Legacy of Darkness -The only element I don't consider canon is Cornell defeating Dracula-
Bram Stoker's Dracula -Since the game was based off the movie which in turn is based off the novel I just had to include it-
Castlevania: Bloodlines
Castlevania: Portrait of Ruin
1999 -Yes, it's not a game, which is the sad part. But nevertheless it is just as important to my headcanon-
Castlevania: Aria of Sorrow
Castlevania: Curse of Darkness was left out as it adds nothing to the series that's worth remembering. Plus (for me at least) it has a whole other slew of issues that just don't work out in Castlevania ie Issac's outfit, St. Germain and his time police/detective aspect, Hector defeating Dracula, devil forge masters, Trevor being stabbed way too easily and then kissed
you get the idea. Left out DoS as it's just more of AoS without advancing the story in any meaningful way.