Firstly, this is a very subjective matter and you should keep that in mind as the comments arrive.
With that said, one would want to know your particular definition of "ultraconcealable" as you have used it. For argument, I am going to assume a gun which can be carried in one's pocket and be virtually invisible to the general public. This might mean something along the lines of a Ruger LCP, a Kel-Tec P3AT, or a Taurus 738 series. While keeping in mind that all of the pistols in this category utilize single stack magazines, which equates to lower round counts (6+1 is the average), the other limiting factor with these "mouse" guns is the fact that they are limited to the .380ACP cartridge. Yes this is better than a hammer, but if you can go up on step, you'll arrive in a more serious selection room.
Perhaps one of the best in the slightly larger category is the Kahr PM9. Chambered for 9mm, it can handle +P ammunition as well. This little gun, and its competitors, opens the door to a better, more serious and more effective carry pistol. Best carried in a deep conceal holster, it can still be easily and safely carried on one's pocket.
Hope this gets you started in your quest and welcome aboard!