If its a modern icon (according to shooters) you're after then Glock (like it or not) or some other striker fired pistol like the Springfield XD line is probably the winner.
If its simply the most recognizable to the public it would have to be something like a Desert Eagle or S&W 500. I would also throw the good ol' 1911 in there, everyone knows what they look like. Like others in this thread have said the 1911 will never die.
Making a good point there... I guess we have to distinguish between guns recognized by actual users for their novelty (polymer, striker, first mini 9mm, PDW caliber etc.) or their reputation and history as duty guns (M9, 1911, SIG Sauer) versus those recognized merely due to their appearance in movies, video games etc. while their quantity of use among actual shooters is neglectable (Desert Eagle, FiveSeven...). Both types can be considered "iconic" in their own way.
Anyway the discussion shows that this issue is highly subjective and I guess we'd get as many different answers as there are users if we'd ask everyone.