The strongest in the current meta due to their effect or synergy with the Minis.
Barbarian is currently the strongest in the current meta due to recent balance changes, and since it can help empower Archer to make it a strong damage dealer to become the team's win condition.
Royal Champion became a very strong pick after a recent balance change that moved her level 8 skill to level 4, making it easier and cheaper to get to its specialty of being able to combo KO enemy units.
Countess works quite well against what the current strongest decks that like to use Barbarian King and Royal Champion with a heavy backline setup. Countess' ability to move around and target low HP units makes it easy to pick off enemies while maintaining some HP.
Considered as the standard, the heroes in this rank don't really have an amazing edge to make them great.