OP asks which Pikachu card to buy, and you basically say every single one of them. 
But anyway, a lot of good suggestions are already given. If prices aren’t a problem, I would just pick up the artworks you like the most. 
In which case you can indeed look at my collection thread, and mostly all my Japanese Pikachu cards, to see which ones are out there (minus the five I’m missing).
One of my personal favorites that isn’t mentioned yet, is the PokéPark PCG-P promo:
Greetz,
Quuador