When it comes to modern pocket pistols, I think there are 3 great choices, depending on your preferences.... If you like revolvers, the S&W 642 is a great choice. There are great self defense loads in 38+P out there. Great pocket pistol, but the cylinder keeps it from concealing as well as the next gun......
If you want the best concealment in pocket carry, the Ruger LCP is hard to beat. Mine has been flawless through hundreds of rounds. It's a .380, but I'm comfortable with modern 380 self defense bullet technology.
If you like the 9mm, Kahr's PM9 is hard to beat. It doesn't conceal quite as well as the LCP, but its hard to beat the fire power it offers.