This card is easily the best (and honestly perhaps the only truly overpowered) Ally ever released. With very few exceptions, you can take practically any deck, cut out a single card, put in a single Treebeard and the deck is probably better overall, even if only slightly. The only real exceptions are janky combo decks that don't intend to actually play the game, so we'll ignore those.
While many high cost characters resemble or even can become heroes themselves, Treebeard is the most efficient cost wise (4 of any sphere) of those kinds of cards. He can boost any deck with Ents by generating resources like a hero and, if you're not using any other Ents in your deck, he still finds use with them by having a built in Unexpected Courage every other turn ... meaning he can hit with his full 4 damage right after doing what he does best, tanking.