Hmm I don't really have just one. I have a few. I'll list 5.
Charizard, I love the Blackish and Red mix on it.
Tyranitar, I don't know why, but I have always loved the color. Maybe because I used to have the card.
Blaziken, Love the darker red on it.
Metagross, as said already, I love that gold and silver coloring.
Gigalith, it looks more natural in color and just looks amazing.