The best Event Card in the game. Period. Widely considered the best Event Card in the whole cardpool.
Arguably (and in my opinion) the single best Card in the game.
A Test of Will in my opinion is the best Card in the game. Beginning with the treacheries (and other "when revealed effects') quests from the earlier cycles, which are often pretty nasty, to the often downright ruinous ones of later cycles, the power to simply say "No" to the Encounter Deck (and at an incredibly cheap cost of 1 resource on top of that and with no strings attached) is insanely good.
I have yet to construct a Deck that contains at least 1 Hero, in which A Test of Will would not be a "automatically include this 3-times" Card.
On the contrary for a lot of treachery-heavy adventures, i have at least considered (and often ended up) putting in at least 1 Hero in each of the players decks (i usually play 2 player games and usually construct both decks), so impactful is A Test of Will.
As an anecdote, i think many players got their 2nd (and maybe even 3rd) core set primarily for more Unexpected Courages or perhaps an additional Steward of Gondor, quite a few of these players might initially not have realised (to be honest both I and the 2 people i played with back then didn't) how impactful the extra A Test of Will would be on the decks performance (and in the case of 3 core sets, the option to include 3x of it it in more than 1 players deck as well...).