I'd like to start a discussion gleaning people's opinion of the two best characters for each sphere. I chose 2 because it can often be too clear on the single best character for the sphere, although there still would probably not be consensus. Also, although this is written with a solo player bias I tried to consider multi-player as well.
(In no particular order)
Leadership: Dain, Theodred. Dain is a king indeed for dwarven decks. The impact he has on them cannot be overstated, particularly in giving utility (willpower) to all cheap allies, which is a great boon to tactics. Also, Dain is a supreme defender (or attacker, or both with Unexpected Courage, which Dain is a prime candidate for). Theodred is in many ways Dain's opposite, good in any deck where Dain only excels in certain ones. It's good to boost Theodred's will (Celebrian's Stone, Dunedain's Quest, Favor of the Lady) as he will probably quest every turn. Theodred doesn't have great stats, but his low threat and utility put him in the top two of Leadership. Honorable Mention: Aragorn (Core)
Spirit: Glorfindel, Eowyn. Glorfindel is Spirit's only high attack hero. In addition to being a great quester with Light of Valinor, Glorfindel races through locations with Asfaloth. He's a superhero with just those two cards, letting him quest and attack on the same turn, and his rock bottom threat of 5 is of course terrific. Eowyn is stellar at questing or other will-related tasks (ie Gollum's escape tests) with the highest will for a hero, and is a handy sink for dead cards and therefore can change the make up of your deck (making it better to include those 3 copies of Steward of Gondor). To me, there really aren't any attachments that beg to be put on Eowyn. (Side note, Spirit has the least amount of heroes, at 7--other spheres have 8). Honorable Mention: Frodo
Lore: Elrond, Beravor. (I really wanted to include Strider here, but ultimately thought Beravor edged him out because Strider needs a deck built around him slightly more and Beravor has lower threat). Elrond even without Vilya is a strong hero, able to amass a multi-sphere army of allies. Besides his Elven Ring of Power, other terrific cards with Elrond are: Warden of Healing, Light of Valinor, Imladris Stargazer, Gildor Inglorion, Unexpected Courage. Beravor is the quintessential hero that's good any deck. Solid 2W, 2A, 2D, 4HP stats, and a great card draw ability. Even though you can't use Unexpected Courage to draw more cards, it's still a good attachment for her to be able to utilize her other stats. I've especially found her good as a defender with Burning Brand. Honorable Mention: Aragorn (tWitW)
Tactics: Legolas, Boromir. Tactics was, for me, the most difficult sphere to determine, with no clear winners or losers. First, the dwarven heroes (Gimli, Thalin) would most-likely have higher stats courtesy of Dain, which could affect their standing. However, I considered the characters separately to get these ranks. Also, I haven't played Hama yet and feel that he could be one of the better Tactics heroes if played well in a deck that highlighted his event ability. I have found Legolas to be a strong and reliable character in that he prevents you from getting bogged down. Blades (of Gondolin, Rivendell) are good on him, as are other attack boosters and Hands Upon The Bow, of course. You will get a lot of milage out of Boromir, who (even more than Legolas) benefits from W/A/D boosts such as the Dunedain cards. Even more, you will have to reduce threat for Boromir, who comes with a hefty 11 threat and will raise it even higher with his readying ability. Also consider ways to heal Boromir because he could be taking a lot of attacks. I like Boromir because he can be a one man army. Honorable Mention: Beregond.
I believe one of the strengths of this game is that none of the heroes are really chaff cards, even Elanor, who is probably the worst hero. She still has many points going in her favor (she is from a very strong sphere, low threat, ability can be very good in certain circumstances).
What are your thoughts? Have your play experiences been different? Weight in!