Gorgorab is the best speed aura arena lead champ when you don't have Abiter. You could use him in dungeon, replace healer Apo. 

Harvest Jack one of arena beast, could use in Dungeons.

Stag Knight, with high accuracy, he is useful everywhere, reduce all enemies atk + def, he is useful even in CB. 

Sepulcher Sentinel could be good, I saw other use her in CB. 

Other champs are good as well, but some of them are best in specific cases, not widely used like those 3 above. 

I'd say depends on what you want to focus on. Harvest Jack is really good at wave control and in arena. Sepulcher Sentinel really good in CB. Apothecary very good in all dungeons, and Gorgorab is similar to Apothecary in terms of making your teams faster, and also has a revive that can come in handy if you don't have any. Stag Knight another one you'll end up leveling up since he's really good in FK and FW