I have a few favorite expressions that I draw
One great one is "Failing to hide that you're excited/happy"
It's super fun to draw and a fun character moment a lot of the time
I'm Also a HUGE fan of the "Oh, you know what i'm talking about!" smile+eye contact, I love non-verbal communication, and this one's particularly fun when said characters are mid-mischief
But as far as "just plain fun to draw" goes, I have to say I love drawing angry characters, Just plain fun to draw, even if it's relatively simple