I've owned or do own every gun talked about in this thread.
Ruger LCP (a direct copy of the Kel-Tec P3AT) is a reliable, small carry gun. Mine has about 1200 rounds down range with not one single jam or malfunction. An absolute great gun and couldn't live without it. It is the gun I carry when I can't fit anything larger on my person to the clothing choice of the day. Not the best defensive caliber, but am confident it will do its job and it certainly has more power than the .25 and .32 mouse guns. Yeah the sight sucks and the gun has a lot of recoil, but this is not a range gun. It is a small defensive gun and serves that purpose well.
Kel-Tec PF9 - Junk, junk, junk. Did I say junk. Kel-Tec guns are reliable AFTER you send them back to the factory to get a fluff and buff by their guns smiths but are pretty much unreliable out of the box about 50% of the time.
Diamondback - worse junk than Kel-Tec.
Kahr PM9- Some people really like this gun. I've owned 2 of them and they were jammamatics and not what I would consider a front line defensive weapon. If you find a good one that works, good, but my experience with them is less than positive.
Smith and Wesson Shield - A great gun. I don't care for safeties on my guns, so I may disable/remove the safety on mine, but the gun is small and reliable.
Smith and Wesson J-frame 442/642/340 or similar - This is the gun I carry most of the time. Disappears into a pocket, 100% reliable, great defensive caliber. Not much to hate about a small pocket revolver. Capacity sucks at 5 shots, so carry an extra speed strip reloader. Many cops on my old department have started carrying these after seeing/shooting mine. They make good backup guns and off duty guns.