I feel that leadership is the obvious answer for most powerful. The Steward of Gondor, a strong dwarf presence, and a general good balance of characters stats make it very tempting for any deck. A lot of times I just assume I'm going to make a leadership deck and then think of the rest of the spheres as backup to round out whatever leadership lacks. Sure it can be argued that all spheres could be looked at in that light, but I think Leadership stands out the most.
That being said, my favorite deck is Lore/Spirit. Being able to cancel effects and heal negates the lack of the powerful dwarf synergy (even though you could feasibly make dwarf decks out of the two spheres; but I feel that focusing on healers, questing, and the notable unique allies is more valuable than tweaking around the lackluster dwarf presence).
Finally I think that Tactics lacks the most as of right now. Quest expansions make it very difficult to have so few characters with notable questing. Creature decks can be hard to pull off and generally rely on a couple unique cards early on to function correctly. Granted there are some very powerful dwarves out there, but them alone aren't really enough to do it for me.
That being said, my favorite deck is Lore/Spirit. Being able to cancel effects and heal negates the lack of the powerful dwarf synergy (even though you could feasibly make dwarf decks out of the two spheres; but I feel that focusing on healers, questing, and the notable unique allies is more valuable than tweaking around the lackluster dwarf presence).
Finally I think that Tactics lacks the most as of right now. Quest expansions make it very difficult to have so few characters with notable questing. Creature decks can be hard to pull off and generally rely on a couple unique cards early on to function correctly. Granted there are some very powerful dwarves out there, but them alone aren't really enough to do it for me.