Although it’s not my absolute favorite Pikachu artwork, I’ll go with the 25th anniversary Pikachu V-Union. It’s so cool to have 25 different illustrators on a single artwork. All 25 illustrators also told a little bit about their Pikachu and how they feel about the anniversary, which was posted on the Pokémon website at the time. @pfm made a great summary of those in this thread.
I will make a separated post with my other favorite Pikachu and other Lightning-Type Pokémon cards. 
Greetz,
Quuador