Saying that some Federalist Papers are "more important" than others gives the wrong impression! Some have more information than others. Some are better written than others. Some seem to be more relevant in 2023 than others.
HOWEVER, with the possible exception of #83, all the Papers give relevant and important information about the United States Constitution. The US Constitution is one of the most amazing documents ever written. It is short, giving the initial impression that it needs "interpretation." However, it is also succinct. The Federalist Papers make it clear that it means what it says.
For example, the Constitution defines the responsibilities of the United States federal government and the powers needed to fulfill them. If you read Federalist Papers #41 through #44 you will understand the reason for and extent of powers, which are:
1. Security against foreign danger. 2. Regulation of interactions with foreign nations. 3. Maintain harmony and interactions among the States. 4. Miscellaneous objects of general utility. 5. Restraint of the States from certain injurious acts. 6. Provisions giving effectiveness to these powers. [Federalist Paper #41]*
Another important contribution of the Federalist Papers is their discussion of how the US Constitution uses the psychology of human behavior to block abuses of power and the growth of tyranny. For example, why does a weak constitution lead to tyranny?
Number 20: United Netherlands: Failure of Legislation for States [paragraph 18] In emergencies, the States-General oversteps its constitutional bounds. It has agreed to several treaties without the consent of all the provinces. A weak constitution ends up being ignored. Then it dissolves either from a lack of power or government officials usurp the powers necessary for the public safety. The usurpation may stop at a healthy point or go forward to a dangerous extreme. Tyranny more frequently grows out of the assumptions unconstitutional power that is needed in an emergency but is not included in a defective constitution, than out of the full exercise of the largest constitutional authorities.*
If you want to understand the Founders’ reasoning behind the US Constitution, if you want to defend the Constitution, if you want to understand why other attempts at self-government failed, if you want to know how tyrants gain power . . . Don’t limit your study to a few Federalist Papers that someone has declared “most important.”
*The Federalist Papers: Modern English Edition Two, 2008