I think as a lone player, unless Fallen_Arises comes to town in which case we usually play 2-3 or even 4 players, that a napolitan ice cream deck is the best, because you get to use only the best cards from three spheres. I read few times that two spheres should be enough, but would it be possible to kill the Witch-King with only few possibilities of cards?
Anyway, I think having Bifur in a three spheres deck is a great asset, even though Bilbo or Frodo may look like a better choice when picking a low-treath hero. A great questing character if he fights alongside with Dain Ironfoot and the player may get ressources faster to play Lore high-cost cards.
So thats my special Hero with whom I win half of the quest (when I win...).
Otherwise, my special card no one in my group played before I proved its efficiency as a finishing move is Common Cause. It may sound a lost because your sacrifice a hero, but refreashing a character without Unexpected Courage in time of need can prove to be a life saver. I'm referring to an awesome 4-players game in Flight from Moria , with Erebor Record Keeper, Common Cause and Ever Vigilant on ERK to unexhaust Dain wielding Abandonned Tools.