Hidden Strength - because no-one in the books rates these heroes, but they are all stronger than they appear. This deck is all about drawing out that strength by piling on the attachments - get an Ent Draught on to Treebeard and you can add 5 damage to give him 8 or 7 . Use Self Preservation, Daughter of the Nimrodel, and Ioreth to heal him up and you can do it all over again next round.
Get Golden Shield and Herugrim on to Éowyn, and she becomes a 4 , 5 , 5 hero, all for a de facto 6 threat! Give her a Steed of the Mark for questing and combat, then either add Unexpected Courage to make her a one-woman army, or give it to Treebeard to turn them into a powerhouse duo.
Bilbo Baggins is the third member of the party, and he is basically support. His extra card draw each round means you'll get to the cards you'll need quicker. His 2 doesn't seem like much, but with an ent draught he becomes an effective blocker, especially with all the healing you'll have. (Throw the third ent draught on a Defender of Rammas, and you have some serious defence.) Add Celebrían's Stone to Bilbo for 3 , and add all the song cards to him (Song of Wisdom is for Eowyn, obviously), which you'll pull with Rivendell Minstrel. Give him Steward of Gondor to pay for everything.
Apart from Bilbo's card draw, there's 2 Lórien's Wealth and 3 Mithrandir's Advice, for a total of 12 cards, and even more if you pull them out again with Scroll of Isildur. Ally Legolas also adds his own card draw ability. Gandalf's Search and Heed the Dream get you the cards you want.