I have seen and heard a lot about Topre's capacitative conical spring/dome switch, which is considered to be very smooth and friction-less, but I think that there may be another rarer older or industrial switch that feels even smoother.
Good scissor-switches can be very very stable -- and because of their construction there is no friction on off-center key presses whatsoever, but they do have a short throw.
What kind of keyboard or switches have you tried, that you found to be very smooth? "Mechanical" or rubber dome, it does not matter.