Most of the responses have been production-type folders, but your original question just asked for the 'smoothest' without qualifier.
The smoothest I have seen yet is my new Darrel Ralph 'Trigger' with his 'Max-glide' bearing system. You can thumb the blade open w/ only a moderate flick of the thumb. Once your thumb learns the appropriate muscle memory, so to speak, opening appears literally indistinguishable from an assisted-opening folder. The blade seems to leap out, with zero wrist action is required. The trick is to push your thumb straight forward, i.e. parallel to the knife handle rather than outward, i.e. perpendicular to the handle.
I've had a few other knives w/ pivot bearings, including several Brad Duncans and a DDR with IKBS. The max-glide definitely raises the ante. I don't claim to have tried everything out there, but I've never seen anything like this. Plus the knife is well-detented enough so that I can carry tip-up with no worries, unlike the Duncans.
I think that most of the DDR 'Triggers' available from purveyors (e.g. True North) did not have the max-glide bearing, so if you want one you probably have to order from Darrel at this point. Also, the Triggers I've seen from purveyors have been tip-down carry only as far as I can tell. Mine is tip up.