Nice idea! (And thanks for reminding me I should post a new Set of Fortnight 
).
Here are my favorite artworks for each of the Pokémon for this week:
Bulbasaur: EX Team Aqua vs Team Magma 39/95
Ivysaur: Intro Pack - Bulbasaur Deck #22
Venusaur: XY123 Promo
Charmander: EX Dragon 98/97
Charmeleon: Team Rocket 32/82
Charizard: EX Dragon Frontiers 100/101 (Gold Star)
Squirtle: EX FireRed & LeafGreen 83/112
Wartortle: Southern Island 15/18
Blastoise: EX Crystal Guardians 14/100
Caterpie: Rebel Clash 001/192
Metapod: EX FireRed & LeafGreen 39/112
Butterfree: Jungle 33/64
Weedle: Base Set 69/102
Kakuna: Vending Machine Series 1 - Blue
Beedril: XY 5/146
Pidgey: Gym Challenge 49/132
Pidgeotto: Theater Limited VS Pack 010/018
Pidgeot: Movie Commemoration VS Pack: Aura’s Lucario 006/020
Halve of the artworks can be found in my National Index collection, where I’ve used the favorite artwork of the cards I already had. Since buying every single favorite artwork can be rather expensive for some cards, there are artworks I like more than the one I actually have in my national index collection. But with almost 900 Pokémon, and an additional few hundred cards due to Mega, Alolan, and shiny Pokémon; Pokémon forms (i.e. Unown alphabet); gender differences; artwork errors; and cosplay Pokémon, it quickly adds up in general without even worrying about favorite artworks yet. 
Greetz,
Quuador