Bucket!
Uh, I'm talking about the one in end of time. Gaspar (?) warns you from using it 'cuz it leads to Day of Lavos. And the only way to beat the game is to smash The Snail. And it can be done only with Epoch, via the bucket, or through black omen (am I missing something? Hopefully not).
So, my answer is the bucket that takes you to 1999 A.D.