Most Irreplaceable Staples?

Which widely used cards are the most irreplaceable? By this, I don't necessarily been the best or most optimal, I just mean least substitutable - i.e., there is no other card in the card pool that you could use a "budget replacement" to do the same thing poorly, because the card is too unique in its effect.

For example - I'm a newcomer to the game, so this might be wrong, but it seems like "Be Prepared" is pretty irreplaceable - there is literally no other card that resets the entire board for both players in the same way. Whereas something like Lady Tremaine - while very strong - has inferior analogues in say, Medusa, dragon fire, etc. Ditto with Maui, who is the best at what he does but there are cards that play similar roles.