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This is what I got to.
When I had to choose, my priority was something like:
-NFE<Legendary<Mega/temp-form<form<non-form/FE
-1<2<3stage line
-least association with unrelated types
-personal bias/mixing generations like they aren't a thing
Meaning I was aiming to pick as close as I could to something generic and typical.
So if there is a legendary, a Mega, or a Rotom-form in some spot...it means that there is nothing more basic for that spot yet.