Oh man, I'm a huge sucker for facial expressions; vayleen can back me up on this. It's pretty much a prerequisite for heirdom for me, as Noro demonstrates in her trip to Bridgeport and Glory shows off in, well, pretty much every picture ever. So this is a really hard choice for me. I absolutely adore the excited birthday face that so many of my Sims are demonstrating up there. And, of course, the proposal rejection face is so adorable.
Gah. Tug on my heartstrings more, kthxbai.
And, of course, it wouldn't be a facial expression post without Luca's epic face of disgust.