With her ranged attack power of 7, the sorceress Yennefer is probably one of the first strong cards you'll come across in the game. As a neutral, the card can be used with any of the four factions. She has the added bonus of being both a hero AND a medic. This means that when summoned, you can bring back a non-hero unit from the discard pile and she will remain immune to special cards and card effects when on the battlefield.
You can obtain Yennefer by beating Stjepan, the innkeeper of Alchemy Inn in Oxenfurt. This can be done very early in the game after leaving White Orchard.