When you reach a certain point, you can't really
objectively say one is smoother than the other, for example, I have TnT's, Obenaufs, Carsons that are as smooth as can be, I know which "feels" most like what I personally think is "smooth", but that becomes subjective and only reflects what your idea of smooth is, I guess we are somewhat limited by words and prejudices.
However, many of the knives that are well known for being smooth utilize teflon washers, which makes it easier to get that silky smooth feel, the Sebenza and Boguszewski's flippers both use phosphor bronze washers and are still silky smooth, so that impresses me a bit more, but, If i had to pick the smoothest, and didn't take into account washer materals, I'd have to go with a good Carson, but Mike is VERY close behind as are Mayo, JWS etc..
Something related to ultimate smoothness is the overall precision of the action, which includes the smoothness, but also the feel of the ball detent as it grabs the blade in, and the "feel" of the lock as it engages. For this, overall precision, that's a tough choice, but the makers that really "do it" for me in this area are: Mayo, Boguszewski, Carson, Obenauf, Tighe, Sebenza and some manual Microtechs. In lockbacks, which of course are never as smooth as liner/framelocks, I go with Dake and Bradshaw.
This is a tough question, and again, made more difficult because we each have our own ideas of what smooth really is.