Favorite card per sphere is somewhat broad. I choose to interpret this as the cards that I enjoy playing the most, rather than the cards that I find most powerful, or use most often.
Heroes
Tactics: Mablung
Why not Beorn?
I still enjoy playing Hero Beorn, but so many enemies have 4+ attack these days that I can't actually use him.
At some point I will put together a deck with Honour Guard and Horn's Cry so that I can keep the big bear alive. In any case, with all of the engagement-based effects in the new Dunedain archetype, Mablung is a lot of fun to play.