Intelligent Life, a The Economist magazine, is asking this question to their readers. Surprisingly, they've not included the question mark as default option (they only shortlisted the exclamation mark, the dash, the ampersand, the comma, the semi-colon and the ellipsis).
They have also asked a couple of well known authors to explain their answers, which is always interesting. Claire Messud's answer (the Semi-colon is great for people who tend to digress) inevitably made me think of Kurt Vonnegut's famous rant against that particular punctuation mark (“Here is a lesson in creative writing. First rule: Do not use semicolons. They are transvestite hermaphrodites representing absolutely nothing. All they do is show you've been to college.”)
You can vote here:
WHAT IS THE BEST PUNCTUATION MARK? | More Intelligent Life
... but remember to come back and let us know your thoughts!