(1) S&W Model 19, 4" barrel; not the smallest thing, but very efficient balance of power, handling qualities, shootability, and concealability.
I can shoot a 4-plus-inch K/L-frame or GP100 better than any handgun on the planet, and a 4" K is notably easier to conceal all day than an L or GP.
(2) Ruger SP101; still powerful, but notably more concealable in some modes than a K-frame.
(3) Seecamp LWS-32; I rarely carry this one, but when the SP101 is too big, I will generally pass the J-snubs over and go all the way down to this superbly-built pistol.
I very nearly mentioned a 1911 on the large end, and a J-frame snubby on the small end. The only easy choice was the SP101.
I actually usually conceal a P229 daily, because this is my uniformed police duty pistol, and is small enough for most concealed-carry needs.
The biggest disappointment was a G27, found to be FAR too blocky for good concealment under a wide range of circumstances, with the clothing I usually wear. I would rather hide a 1911 or P229 than a even a "baby" Glock. I can hide barrel/slide length, and height as measured from base of grip to top of slide, well enough, but the rear of a Glock's slide, coupled with the blockiness and width, is a "tell" I would rather not deal with. I sold my G27 to a co-worker with a chunkier build than me. He had one G27, and wanted another, for his UC narc work. (We buy our own firearms, at my PD.)