If you don’t get it, you don’t get it, and that’s fine.
From someone who’s went through film school:
It’s a great movie. I wouldn’t call it second ever, but it’s up there. The cinematography is some of the best put to film, the writing is excellent, and the acting is phenomenal. I love the music as well, personally. Mafia stories are/were big hits for film in general, dating back even to the black and white era.
In simple terms: basically everything about it is made better than your average film, and if there’s something you specifically like about films (music, cinematography, etc.) It’s usually an easy example to point to for a quality example of said thing
But yeah, there are too many movies in existence to put any real stock on “x best movie of all time” things