I think A Woman of Affairs (1928) is my fave silent of their partnership though as an overall package for script and performances Queen Christina (1933) is the best film.
I go with A WOMAN OF AFFAIRS as it my favourite Garbo silent anyway.
As much as I love Queen Christina, I don't think it's one of John Gilbert's better performances. So I voted for A Woman of Affairs. Which is one of my favorite of Garbo silents (along with The Torrent and The Mysterious Lady)
Flesh and the Devil (1926) is my favourite Garbo movie, there is soooooo much chemistry between them, especially the smoking scene.