I'd say the most carried gun on the streets of the US is going to be a .25/.22 of the Davis/Jennings/Raven variety.
The folks on this board (and probably many of their firends) are into guns. I think that many of the responses of "Glock, etc" reflect that. Many of my friends that are not into guns, but carry because of security concerns, will actually carry small frame revolvers or small .380 autos (like the Bersa, PPK, FEG, Makarov, etc). Sure, many will say that if they're really concerned over security then they need to carry "X" brand. But the fact is that most people that carry a gun, do not get into getting all "geared up" with a large frame auto, holster, spare mags, mag holders, flash-light, pepper spray, and whatever else just to go to Food Lion. They just grab a gun with their car keys, and they want it to be just as convienient as just grabbing those car keys.
The other fact is that most people would rather not spend several hundreds (or more) of dollars on a gun. I don't mind spending money on guns, and I'd venture to say most folks here don't mind it either. But most folks don't buy higher-end guns anymore than they buy higher-end tools, or higher-end cars. Again, that would result in the small "Saturday Night Specials" as what I think are the most carried.
Personally, I carry a Colt Detective Special .38 revolver. I try out all sorts of others, but I keep ending up with the Colt.
Ross