For me, it is the Kel-Tec PF9. I really want to like it, because it's a nice size and weight for a carry gun but I just can't get it to work for me. No matter how I hold it my hand takes a beating every time I fire it. I tried straightening the trigger, different grip sleeves and different types of ammo.
However, I can shoot the Taurus TCP with no problems whatsoever. I have one and my daughter also has one, and I can shoot either one reasonably well. None of my other handguns are particularly hard to shoot either.
My nephew had trouble shooting it also, but a one of my shooting buddies was able to shoot it quite well. So aside from a little ammo sensitivity, the problem is with me and not with the gun.
So for now the TCP is my pocket gun and the PF9 has gone to a new home, traded straight across for a Ruger P85 plus accessories.