Sometimes I can remember a single look on the face of a cartoon character just as easily as that of a real actor.
Some of my favorites:
The look on every other Futurama character while Bender reads his sickly-sweet greeting card for Mom.
The look of horror on Stewie’s face when he realizes he was just nursing from Peter’s nipple.
The impound lot owner from Shelbeyville: after he eats the Springfield lemon, the look of watery-eyed triumph.
Milhouse’s teary-eyed expression as they tear down the candy factory (that mirrored that famous shot of the Frenchman during Germany’s invasion in WW2).