Base: Pikachu (duhh… 
). But as for actual favorite artwork, probably pretty cliché, but Charizard
Jungle: Jolteon for sure, no questions asked
Fossil: I always kinda liked the Ditto artwork; although Omanyte is pretty good as well
Team Rocket: Dark Golduck
Gym Heroes: Rocket’s Snorlax or Sabrina’s Venonat
Gym Challenge: Japanese Misty’s Tear 
Rofl, jk. Hard to pick one of these, but probably Giovanni’s Pinsir
Greetz,
Quuador