It's amazing now days how well formed animation is that we can actually tell what a character is thinking by the look on their face. I especially love that first one.
My mind is too jumbled with cartoons to think of specific expressions without watching a bunch of them. But we do have easy access to Foster's screencaps!
So I searched through them and it didn't take long to find some of my favorite moments having to do with expressions.
Mac's unamused look is such a classic. He does this a number of times, and I love it each time.
And then there's his sweet "Pretty please?" look. Makes him appear more like a little kid, when many other times he seems more mature for his years.
Blooregard gets sooooo excited and enthusiastic sometimes! I love his expression here, lol! And paired with Mr. Herriman's unamused look makes it even better.
This has got to be my favorite Wilt expression. It's best when Ed is actually hugging him, but you can still see it here. He looks so tired of the nonsense going on, and annoyed at Ed's behavior.
And Frankie's, she does this a number of times too. You can just see how she's thinking of something mischievous, and maybe an evil laugh in there somewhere too, heheheh. I love the look she gives Mr. H when Coco's trying to talk him into letting her have the room in "Room with a Feud" too.
Edit: Here's Frankie and Coco. Both their expressions here are great.