It's very rare I play mono sphere, but its probably Lore.

Mono Leadership. It's the most viable one, I think and can become really powerfull in both quest power and combat prowess!

I've definitely played more mono-Spirit than the others, but that mostly on the strength of Caldara decks.

In the abstract: Probably Lore over Leadership. Leadership has too many resources and not enough card draw, which means you get to play a lot of big, expensive cards, but you don't see a ton of your deck. Lore has the draw, and a lot of fun cards to boot.

Just get 3 copies of Erestor and King Under the Mountain in your monolead deck and you'll be fine.

Mono Lore is probably the only Mono-Sphere I haven't tried but to rank them:

Spirit

Leadership

Tactics

They're all really fun in different ways so it's hard to pick just one!