As the title says, who is your favourite ally and why? If it's too hard you can break it up by sphere. But still try to give an overall favourite.
Tactics: Eagles of the Misty Mountains - Really wanted to say Boromir or Legolas but honestly these beautiful birds are what makes my favourite play style possible and grow into an amazingly powerful force of nature once a few of their kin join them.
Spirit: Zigil Miner - when I'm not flying with Eagles I play a lot of Gandalf hero, and these guys are staples of that play, being able to turn your deck that's just sitting there into cold hard cash now! Runner up the new Wild Stallion.
Leadership: Wiglaf - the newest ally on my list but this guy is quickly climbing to the top. Epic Viking-style art, his high cost is partially offset by being able to get a free item played on him, and once you start putting attachments on him, he becomes better than most heroes. Seriously just add the included-in-the-same-pack Wild Stallion and Ancestral Armor and you have a 3/3/4 hero that can do everything for you, quest, defend and fight. Just awesome!
Lore: Warden of Healing - Really hard to narrow down a single Lore ally, Henamarth, Gleowine, Gildor, and Wellinghall Preserver, wandering Ent, to name a few are all amazing allies, but being able to heal 2 separate damage off of anyone each round is just so awesome for his cost, even better when combined with Elrond.
Neutral: Treebeard - Gandalf is classic, Saruman is fun, Rangers are good multi-role troops but nothing compares to the awesomeness that is Treebeard, and as much as I love his hero incarnation, this version is actually better. Solid ally that is in every single force I make.
Overall my pick for favourite ally has got to be Treebeard, he may not be as flashy as the new stuff, or as classic as the original tried and tested stuff, but dang he sure is tough and a great ally to whatever fellowship I am assembling.
So what's yours?