Greetings Nemanja,

Welcome to the Collector's Corner, and you have asked a very astute and productive question. It is not easy to give a "perfect" and/or definitive answer . . . there's no set-in-stone standard(s) as there are for comics, baseball cards, etc. There is also not /universal agreement/ on which NGM issues are 'most valuable' and 'most important'. 

Back in the day there was a "dean of National Geographic collecting" named Edwin Buxbaum. He wrote/published 4 "collector's" guides to NGM's + Society material in 1935, 1956, 1962, and 1971. In 1982 another gentleman --Roger Nathan-- published a most-excellent source for materials & publications. Since then, it's been adrift, if not a free-for-all, amongst collector's and enthusiasts as to both concurring assessments (opinions) as well as any one individual having all the information at-hand. Post-year 1998, it is nigh impossible to document and account for all, any, and every item produced by the Society, etc.

Note: there was also another good fellow by the name of Don Smith who also lovingly compiled Nat Geo info and lists, etc. He published in booklet form, several guides between about 1978-2000. 

                               * Messieurs Buxbaum, Nathan, and Smith are also all now-deceased, sadly.

Having said all that, might I make a suggestion? It will at least give you a place to start . . . 

Our friend and fellow 'Corner member Melvin De Vilbiss has written up 2 highly regarded & appreciated overviews and statistical surveys about NGM selling-buying-collecting (including proposed 'grading' standards). Here are 2 links to begin ---->

RE: guidelines to selling/buying NGM's - -

http://ngscollectors.ning.com/forum/topics/a-guide-to-selling-natio...  

RE: 'grading standards'- -

http://ngscollectors.ning.com/forum/topics/proposed-ngm-grading-sta...

Also, I am attaching a file to this response. It is the 1971 version of Buxbaum's "100 Best Special Numbers of the National Geographic Magazine". It gives you an idea of NGM's from 1911-1971 that were 'important', groundbreaking, and in-demand during those years and decades.

The first PDF attachment is a bare-bones copy of the 1970 list; the 2nd Word document attached is my modified version of the 1970 list - it has extra notations and/or footnotes for various issues.

Top 100 NGM issues (c. 1970)

Buxbaum's Top 100 NGM's (c. 1970) ~ with my extra notes (2017)

Very cordially yours,

        ~ Scott Shier