the smoothest sounding speaker

Low distortion
Wide even dispersion
Flat frequecy response
Well Damped and Excellent transient response ( & no dull compression)

The speaker that I have found to strike the right balance between resolution and smoothness is the Merlin VSM-MXe. The other speaker that seems to be as good are the various speakers I have heard from Verity; if I didn't own Merlin, that would be the speaker line that would most interest me. I also owned the Vandersteen 3As/2wqs, but the Merlin was an improvement in the area resolution and micro dymamic detail. The Merlin uses the Dynaudio Esotar tweeter and I imagine that is a reason for its smooth upper frequencies. Of course, using tube amps doesn't hurt.

For example here is a distortion plot and cummulative spectral decay of a mid range driver. Nice because it is playing at 96 db SPL at 1 meter (that is real world music loud - so it is playing at useful levels not soft levels for the test - this takes care of dynamics cleanly).

Distortion and Cummulative Spectral Decay (CSD) Note the CSD goes to -35 db which is an entire 10 db lower than you usually see on Stereophile plots - and remember you can ignore the breakup/resonances above 4K as this would be removed or taken care of by a crossover (it is a mid range driver - it can do tweeter duties)

The dispersion is nice and even also - see Dispersion plot

I own Hyperion 968 and I believe it's very smooth sounding speaker. Not to the point of boring like some British speakers, though...

another vote for spendor and aerial