I don't have a bunch of them unlocked, but Athena from the 9 star offer is pretty good. She's really tanky so she can help hold back some of the more annoying bosses, and both her passive and active skills deal good aoe damage.

She does cost 400 gems, but it's not too hard to get that between the ones you start with, some from the early star chests, and just burning some energy on the first stage heroic and hell modes for the missions to get the gems.

The only real downside is her mediocre dps outside of her skills and the fact that you'll need to farm mission 16 to get more shards to level her skills.