Golden rule of canon: If I like it, it's canon.

Outside of the one in my head, all canons are worthless. Since that's where I exist, it's the only one that can logically count from my perspective. Though I usually prefer things that aren't enough of a cluster**** to allow large scale canonicity arguments to become a thing (such as superhero comics or SW, both of which can gtfo as a whole, minus the bits that I like).

Continuity is a bitch and a half though, especially when contradictory yet both awesome things X and Y exist and you can't decide which is cooler and should be the one that counts. Even in that case though, they're both canon.
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I also used to nerd out about all this canonity/continuity crap before reading Umineko no Naku Koro ni and thinking about its themes a bit cured me of this disease. It was like magic.