I'm new here, and this thread has made me kick myself for not including All About Eve in my top 5 films intro list.
From the first viewing I was struck by the dialogue and the wonderfully ruthless sides of both Eve and Addison, summed up in this exchange:
That I should want you at all suddenly strikes me as the height of improbability... you're an improbable person, Eve, but so am I. We have that in common. Also a contempt for humanity, an inability to love or be loved, insatiable ambition - and talent. We deserve each other.
(quite similar to a line of Daniel Plainview's in There Will Be Blood)
This is a personal thing though, as I always go for the ruthless manipulative characters, The Sweet Smell of Success makes a good partner to All About Eve in that respect.
Lastly, the ending, the final shot, is one of those few perfect cinematic moments where more is said with a single image than could be with words. Sure we get that the young girl is manipulating Eve in a similar way to how she got to Margo, but those mirrors condemn all those who play the game, even the young girl just starting will have it happen to herself one day.
I suggest all who've seen this watch No Way Out if you want to appreciate Mankiewicz - both great films done in the space of a year, but completely different in style. He's all too often overlooked.