In the story "A Christmas Carol," which spirit taught the most important lesson to Scrooge?

Each of the spirits teach Scrooge valuable and important lessons. Personally, I think that the first spirit and the last spirit teach him the most important lessons. But, if I had to choose between the two of them, the last spirit probably has the most impact on Scrooge and his behavior going forward.

It is worth noting that Scrooge begins to immediately see some of the error of his ways with the first spirit because...

Each of the spirits teach Scrooge valuable and important lessons. Personally, I think that the first spirit and the last spirit teach him the most important lessons. But, if I had to choose between the two of them, the last spirit probably has the most impact on Scrooge and his behavior going forward.

It is worth noting that Scrooge begins to immediately see some of the error of his ways with the first spirit because after seeing how Fezziwig treats his employees, including himself, he says he wishes he could say a word to his own clerk. That demonstrates that he realizes he is a bad employer as he looks on at the good employer he once worked for. That is the first of many valuable lessons he learns.

However, his most important lessons come in Stave Four when the Ghost of Christmas Future shows Scrooge his own death. Scrooge panics and asks the spirit if this is certain or only possible:

“Men’s courses will foreshadow certain ends, to which, if persevered in, they must lead,” said Scrooge. “But if the courses be departed from, the ends will change. Say it is thus with what you show me!”

Scrooge wants to know whether, if he changes his ways and if he learns from his mistakes, his future will change.  The spirit does not answer him and when Scrooge awakes, in fact he DOES change. It appears that this final spirit had the most impact on Scrooge and his life choices, making his lessons the most important of all.