This is what you've been waiting for -- the two best cards in the game! We'll start with none other than wonder girl Ciri.
Ciri is tied for the highest attack rating in all of Gwent, with a staggering 15 melee points. She is a neutral card, so even the monsters can get their hands on her.
She adds a huge amount of strength to any deck. Her sheer power can change the course of a match and tip the balance when you are struggling against a good opposing draw. Don't forget -- as a hero, she remains unaffected by special effects, so once she's summoned she is guaranteed to deliver 15 points to your total score.
You can obtain Ciri by beating the Scoia’tael Trader, located at a camp in Novigrad Forest, at the end of the Gwent: Big City Players quest. In theory, you could do this quite early on if you concentrate on collecting good cards and progress quickly through the main storyline.