"Most powerful card in the game"

[ForestRunner]

On the Zombie side there are cards which can make short work of the heroes as well, so it isn't always in favour of the heroes. Just two potential gamebreakers from TP:

Vicious attack: Roll an extra fight dice. If the Zombie wins the fight, they do an extra wound to the Hero (student instakill).

Unrelenting Attack: Play at the end of a Fight with a Zombie to let that Zombie fight again (this is NOT a Fight card so can be lethal in combo with one).

Twice now, I've killed off Nikki on turn 1 without her even having the chance to move with the cards above (once each) combined with movement cards (Relentless Advance/Shamble). The spawn pit adjacent to the Tavern also does Nikki no favours in this regard.

The pistol and police shot gun can be dealt with using Out of Luck or Pulled Away as the wording on the additional ammo card is specific around the ammo check rolls rather than discarding a token (or card itself) instead of the gun item when it's lost.

Above said, the actual question can be difficult to answer about the most powerful card. I believe any card can be powerful dependent on the situation, especially for TP, not to mention some of the combos you can put together for both sides.