the smoothest sounding speaker

Ariel 20Ts are incredibly smooth, and also detailed and dynamic. Definitely worth a listen at the 20-something pricepoint. I heard them with monster mac amps and they were smooth as butter

Silverlines are very smooth, when at CES last year, Alan's Silverlines were 1 of those rooms that seemed effortless, a nice place to relax with a glass of wine.
I my room, they are the smoothest I have had, and with my hardwood floors, with rug down, I need the smoothest.

If you are using the components described in your system, don't blame the speakers. You need to look elsewhere if you are experiencing listener fatigue, mainly your source and amplification.

Oz

A speaker can be made to sound smooth by designing in a frequency response dip in the lower treble region (somewhere around 2 kHz to 5 kHz). The Silverlines mentioned by Zapper and Bondmanp often have a pronounced dip in this region and I think that Shahinans do as well, and a lesser dip is present in many British speakers (and I would guess something like that is present in most of the other speakers mentioned in this thread that I'm not familiar with). Maggies have a broader, gentler lower treble dip that smooths the presentation.

As Mrtennis points out, there are tradeoffs involved. To my ears, a speaker with reduced energy in the lower treble region is lacking in upper harmonic energy on many acoustic instruments, and so it doesn't have as much "life". I find myself wanting to turn the volume level up louder to hear the harmonic richness that I expect.

I used to own Quad 63's, and would not describe them as an especially "smooth" and forgiving speaker - they could sound somewhat aggressive with some program material. Quad 57's are smoother, and I haven't heard the latest Quads.

Speakers that are free from peaks in the 2-5 kHz region (whether on-axis or off-axis) are likely to sound fairly smooth without trading off too much upper harmonic energy. But if you want a speaker that sounds smooth even with a harsh recording, you'll need a speaker whose response dips signiifcantly in the lower treble region.

Duke

Zu Definition MK2.