Brainthor when Flash was going around the world to punch him was the best. He was so smug, and then he turns around and... "What the...oh SHIT!!!".
Originally posted by Omega VisionBlack Adam after punching him.
Pretty much any face made by anyone when Superboy Prime shows up angry.
Not really an "Oh shit" face but I really liked the panel in Siege where Captain America throws his shield towards Osborns face and it said "incoming!"
and:
Wolverine when he has to fight Spider-man.