I'm required to carry Glocks, in .40 calibre. For "uniform" or "open carry" duties, I like the Glock 22. Of course, I carry 15 rounds, +1 in the chamber... To me, it fits my hand, and it's a comfortable gun. Currently, I use a Streamlight M3 Illuminator mounted on the rails; the Model 22 is big enough that it's not unbalanced. I suspect that a Model 23 (slightly smaller) might be. That said -- the Model 23 is a good fit for lots of people, and it's a lot easier to conceal than the Model 22. (You can conceal the Glock 22 -- but it ain't easy!)
For concealed carry, I use a Model 27, with the grip extender on the magazine. I can shoot reasonably well without it... but it's definitely a little easier to control the gun with that extra "finger width." It's a very concealable gun... Most folks don't even know I've got it, and I tend to use either a pancake or paddle holster on my right hip.
I like Glocks; they're "cop simple." No extra buttons or gadgets to work with, unlike Sigs, for example. It's just ESSENTIAL that you keep your finger off the trigger with 'em unless you want the gun to go "BANG." And Glocks WILL go bang... I think it was on MT that someone posted a guy's blog about his attempts to abuse a Glock and keep it from firing. They're very tolerant of not being cleaned often, of being carried in bad weather (and concealed carry might as well be bad weather; think about the lint & humidity at close to body temps!)
That said, someone was telling me that there's a gun out there (I think it's made by Springfield Arms) that's got most of the good parts of the Glock, plus a grip safety. I kind of like that idea, since there have been incidents of Glocks firing when stuff (like coat drawstrings) got caught in the trigger as the gun was holstered.
Of course... if you really just wanna go "bigger for fun"... try this:
