Thank you for this thread! I'm a huge fan of Muppet facial expressions, they're so awesome.
I also love Wembley's expressions, especially the nose/mouth one that Slackbot mentioned. It's got so many different emotions packed in it and sometimes even can be used for different emotions, like, being shocked, scared, embarrassed, etc.
One that hasn't been mentioned is Elmo. I love it how Kevin can push down on Elmo's mouth and make his eyes sink in. Hilarious for showing extreme frustration.