Hi there. I actually have a completed National Index collection including all forms and Shiny Pokémon, which I posted about here. Unlike you, I didn’t just want the first time the card was presented on a card, but instead went total random. I primarily selected the artworks I liked most from the cards I had or bought, or were most affordable (for the Shiny Pokémon mainly). I also collect all 14 languages, which can be seen in my National Index collection.
As for your actual question, I personally ordered each gen separated like this (which you can also see in my article):
All regular formats of the Pokémon come first (i.e. here the page including a general Unown artwork):
After all the regular Pokémon, I had all forms in this order:
- Gender differences
- Mega Pokémon forms
- Alolan Pokémon forms
- (not added yet) Gigantamax forms
- (not yet added) Galarian forms
- Shiny Pokémon
- Combination of Shiny & other (i.e. Shiny Mega, Shiny Alolan, etc.)
- Official Pokémon forms and variations (this category would include the Unown forms, Deerling seasons, Deoxys forms, Primal Kyogre/Groudon, etc.)
- Unofficial Pokémon forms and artwork errors (i.e. Blaine’s Charmander with his back-horn, Fossil Magneton with the bottom Magnemite upside down, Jynx when she still had a black face instead of purple, etc.)
- (not added yet) Cosplay Pokémon (the cosplay Pikachu & Eevee basically)
Greetz,
Quuador