When I was State Cop back in the 80's, I always carried a Freedom Arms 4 shot mini .22 mag revolver, usually on a dog tag chain around my neck and under my shirt. Smallest, deep concealment, "get off me" gun I could find. Kicks like a little mule, and spurts a foot long flame. It was a last ditch emergency gun, and only that. If you took the time to actually line up those little sights, you could hit a paper plate at 15 yards, most of the time. My other back up, the one to go to if I lost my duty weapon somehow was a PPK's, that always rode in my pants / coat pocket.
I have shot a few derringers, but never owned one. Although the larger caliber is good, they are just not, in my opinion, a good choice. To large / bulky / heavy for the 2 shots they offer, and hard to hit with. There are better choices around. The kel Tek .32 weighs, if I remember right, like 6 ounces, is flat, and easy to hide. Reliable. It is easier to hit with then a derringer or mini .22, and the .32acp, loaded with good bullets (cor-bon, federal hp) is a decent performer. I have one, and I carry it in the summer when wearing shorts and a Tee shirt, and anything larger would be hard to conceal. By the way, I have huge hands (size 19 ring is tight) and I have no problem with my Kel Tek. The mini .22 mag was the one that was hard to handle. Anyway, there are several other good choices out there in this size and caliber range. But to answer your question, ultimate concealibility would go to the mini .22.
Larry