Katara, Water Tribe's Hope

Waterbend is the piece of engineering worth studying, because it overwrites rather than adds. Most anthem effects push your team above baseline and never touch the ceiling; this one replaces base power and toughness entirely, setting every creature you control to a single predictable X/X before the layer system reapplies whatever counters and static pumps you have stacked on top. It is a floor as much as a ceiling: your one-drops and tokens all become X/X, so the mode where you fund a large X buffs the small end of the board into real bodies. The convergent-cost structure is where the decision lives. Artifacts and creatures each pay for toward X, and crucially the permanents you tap to fund it still receive the buff, so a creature can be both the resource and the beneficiary in the same activation. That is what makes Vigilance sing rather than a stat you note in passing. Katara attacks and stays untapped, so mid-combat she is still available to tap for toward her own Waterbend, converting the attack into fuel without leaving the assault. And the timing clause reads "activate only during your turn," which permits instant-speed use across your own combat: hold it up, let blockers commit, then set the whole board to whatever X you can afford after seeing the damage math. The enters trigger drops a token that feeds the same equation, one more of toll the moment she lands.