There are three card games that will get players:
1. Scrolls - Win/Lin/Mac - Paid
2. Faeria - Win/Lin/Mac - Paid - ( closed beta )
3. Hearthstone - Win/Mac - Free - ( closed beta )
Scrolls & Hearthstone are more casual than Faeria.
Faeria got more than cards to play, You have to shape the terrain & keep economy ( gold & faeria [ kind of mana ] ).
I think that Faeria will be the best card game of this/next year.
For best, check them all & choose