All you need to know from RMA's lecture is that Miracle Man 15 is an undisputed "classic cover" and ASM 300 is not.
Oh wait..
#1 Here:
https://www.google.com/amp/s/sprucestreetcomics.wordpress.com/2017/06/27/the-top-25-greatest-amazing-spider-man-covers-of-all-time/amp/
#6 Here (referred to as a classic in introduction):
http://www.nerdgoblin.com/comic-book-rewind-top-10-spider-man-covers/
#23 Here (referred to as a classic in introduction):
http://www.comicsbackissues.com/best-spider-man-covers/
#11 Here:
https://www.google.com/amp/s/www.cbr.com/the-15-most-iconic-spider-man-covers/amp/
Heritage Auctions seems to think it's a classic and "iconic" cover as well:
https://comics.ha.com/itm/original-comic-art/covers/todd-mcfarlane-amazing-spider-man-300-cover-original-art-marvel-1988-/a/7099-92276.s
One thing of note about all of these lists- they do not all contain the identical books in the identical order.
Go figure.
Now, just for fun let's have a look at some synonyms for "iconic":
https://www.thesaurus.com/browse/iconic
Two words included on there: "classical" and "historic", the latter which RMA used to describe the cover to ASM 300 in his first post on the matter, incidentally.
Can't wait for this movie!
-J.