Criteria: Cards which allow you to pay energy for an effect that has an effect if it is activated multiple times, that does not result in just more energy (get outta here, Aethertide WhaleAethertide Whale!), and is repeatable in the same turn with a cumulative effect (get outta here, Hightide HermitHightide Hermit!)
Teddiursa evolves into Ursaring at level 30, offering a significant boost in stats and utility in battles
Donald Duck, Pie Slingers secondary ability is also very strong: if your opponent has 10 or more lore, this cards strength as in, the damage it can deal in challenges increases to 9
Seedborn Muse has an extremely powerful ability, which allows you to spend your mana on Morphs during your turn and then spend that mana again to unmorph your creatures on your opponents turns
Not every deck is going to be able to play Field of Ruin