Lore.. no question.

Lore has the most powerful effect in card games, card draw / deck manipulation. Any deck, of ANY sphere is smoother with a lore splash. Lore improves any deck it is in.

With the addition of the traps and ranger subset they are now even able to handle large attack needs on their own. The last remaining weakness. They quest, have great support, and most importantly even beyond card draw is they are the sphere for encounter deck control. Oh yeah, they also have the heals.

Control decks rule.

If there was a sphere I would call a Generalist sphere, then it is lore.. dose a bit of everything. Great for solo players.

For myself, I tend to play Leadership. I enjoyed the Gandalf-Sneak Attack combo. That brought me deeper into the sphere. It wasn't until I realized that resource management cards like the Steward of Gondor allowed for mighty, powerful armies. When I play, I usually find myself amassing a large group of allies while the other player I usually play with is struggling to just pay for a Citadel Plate. They're also the best at being ready to drop Gandalf in the Core Set un-Sneak style.

Leadership seems like it is the best of most worlds. I admit it lacks when it comes to threat management, but what sphere is perfect?

I like all the Spheres, but play Spirit the most, mainly because there are a ton of little decisions to be made all the time.