Since the top 10 I did earlier only contained cards from my personal collection goals (Pikachu, Seviper, FA Supporter/CHR cards), and also doesn’t just focus on artwork, but also on other aspects (e.g. the Japanese unlimited WCP Seviper because it was the last Seviper I needed to complete my Seviper TCG collection, and is also the rarest Seviper card by far; the Pikachu V-Union Jumbo card because I thought it was very cool and special to have a single card done by 25 different illustrators for the 25th anniversary; etc.), I decided to post another list below solely focusing on artworks, and without any cards from those earlier mentioned collection goals.
Of course, the first one remains the same, since the Moltres WotC #21 promo is my favorite artwork for the Pokémon TCG of all time. 
This list was a lot harder to narrow down to just 10.
Honorable mentions:
Greetz,
Quuador