Smallest I have is a Radom P64... loaded with 6+1 Hornady JHP 9mm Makarov rounds. Usually carry IWB but can also go in-pocket with Nemesis holster in cargo pants. Similar in size to a Walther PPK; ankle carry is apparently ok but I've never tried it. Shoots straight & the Hornady ammo goes through three gallon jugs & mushrooms pretty well.
My deep concealment gun currently is a Taurus 85UL with Crimson Trace grips. I have gone through a lot of hassle to find something better, even buying other guns, yet somehow I keep coming back to this one when I need to carry something that I don't want to be discovered carrying. It's light, small, SUPER accurate with the laser grips (It's really difficult to express just how accurate it is), and it carries a caliber that I am comfortable with. I can carry it on my belt, or on my ankle, and it just doesn't get on my nerves.