>>32514297I built my living dex on gen 6, so I transferred everything I needed from gens 3, 4 and 5 up. I pooled Pokémon together from 12 games to get 721 Pokémon, then banked the lot up to gen 7 when we finally could.
I like keeping as much as I can on Pearl, because that game is important to me, but at the same time, I was also happy to move evolved Pokémon and Japanese things from the GTS up to gen 6 because it made my life easier.
So basically I transfer everything I feel like. There's no pattern.