9mm.....I have three.
CZ75B: Love it. Solid, all metal gun, extremely reliable. Points well.
Ruger SR9c: Very nice little gun. For me personally, this one is the most accurate, and small enough to conceal without being so small I can't get a good grip on it.
Sig P938 SAS: Highest price of my three 9's, but a well-made little package. Conceals very well; in fact, lots of times I just slip it into a pocket holster, and it disappears. Accurate, solid, and I really like the 1911-like feature set.