The answer is that they all are, for different reasons.
Everyone strives to attain full light in Masonry. For this alone many will answer that The Master Mason Degree is the most significant. It is, after all, the degree, where we do business, vote, and share all the wisdom of all three degrees. And once a Master, everything in the lodge is open to you. You are a full voting Lodge Brother, eligible to hold office.
A Brother who loves the lessons and history contained in the Fellowcraft Degree might argue that no one could possibly understand the true nature, beauty and history of our Fraternity without the content of this degree and the nuances contained therein.
I would ask you to consider the Entered Apprentice Degree from the standpoint of the new EA. What could be more important than the initial commitment to become a Mason? That first step into the world of Masonry and life as a Masonic Brother is most significant. Without it nothing else happens. The EA holds our future and he must be celebrated. Close Mentorship by his new more informed Brothers is critical. Statistically, this is also where many of our new Brothers fall by the wayside before they ever take their second step in Masonry. We must not fail our newest Brothers. It is up to us to focus our efforts on bringing him into the life of his Lodge.
By the way, just when you are comfortable that you have all the answers, talk with a York Rite Mason who has completed the Mark Master Degree. I suspect he could make a pretty good argument for that degree too. He may point out that this Degree used to be the fourth degree in Blue Lodge. You may not know what you are missing. The one truth I have found is the more light we get, the more we can see.