The best speaker you ever heard?

Sonus Faber Elipsa, all the better for being driven only by a McIntosh integrated, and a McIntosh CD player.
Former benchmark was the Vandersteen 5, with VTL amp & preamp & a Levinson player (Audio Concepts in Houston, RIP)
Recently heard the 2nd generation B&W 802 diamond with McIntosh 601 monos; very nice. Heard Cremonas in same room at same time, Diamonds very impressive.
My Thiel CS5's, connected to a Krell FPB 300 via Threshold FET 10 e & PS Audio Lambda, ain't too shabby, for a twenty year old system!
AudioAeros, at Audio Concepts, were very impressive as well, come to think of it...

Probably the most striking speakers that I've heard were the new "Pure" series PureVox speakers from BMC. While I sat there, they just picked them off the display; placed them on the floor in front of their other big speakers; hooked them up; and played them. Shockingly good sound even then, but after a day or two they were awesome.

The ones I enjoy my music through every evening - Zu Druid V's with Dueland Cap upgrade.

I have to update my answer from the MBL 101 Extreme to the Rockport Altair. I heard the Altair at the Sound Environment in Omaha, Nebraska. They did things that I couldn't believe were possible. I have never heard their equal.

Mot

I've never heard speakers over 50k that sounded that much better than many under that price point. WIsh I had a dealer that had some of the esoteric speakers on display locally.