In the OG days my favorite was the Exploding Dwarf. I remember placing him in first position and hoping that combined with a fast lineup I would win before my opponent knew what hit them. Once or twice I defeated my opponent before any of their cards got a chance to attack.

Currently, my favorite card is Halfling Refugee. It is a neutral Rebellion reward card. For just 1 mana it is incredible with a lot of versatility. Lots of health plus self-heal and a decent melee attack. I work it into almost all of my small and medium mana battles.