Sean Penn
Why: God, this guy is just so serious! Lighten up, man. You're a celebrity, not the Mayor of Stodgytown. Then there's that terrible rage, especially toward the paparazzi. A few tokes will take care of that. Also, with all his gallivanting to do good in New Orleans and Haiti, he could probably use something to take the edge off.
Method: A good, old-fashioned pipe.