The best speaker you ever heard?

Kraco 6x9, 2 ways.  It's all in the cables. I used lamp wire for one and a $42,000 solid core meteorite sourced ore with penguin foreskin insulation on the other.  My Fiero never sounded so good!

The revel salon2 was the best sounding system I heard til I bought them and brought them home. now I disagree with the OP and while speakers are important all the pieces of the puzzle have to fit in a hi rez system for it to be musical.

Bingo Steve.  Currently, the "best" speaker I have owned/heard is the Trenner & Friedl RA fronted by a Traumatic 300B amp.

Tannoy Westminster driven by Cary 805 monoblocks.  Not my system but it's what turned me onto Tannoy and Cary and now I own lesser models of each.  Even the lesser models are nice.

I also love JBL L300s, and believe it or not the Paradigm Signature S6 speakers that I have sound better than any other speaker that I've tried in that particular system/room.  They've lasted longer than any other speaker that I've owned.

The best system I heard is mine  designed partly with my own cleaning methods, this is the very important part, cleaning is the most underrated part of any system at any price... 

Mission Cyrus 781 speakers+Sansui Au 7700+ starting Point Systems Nos dac… I am envious of none other now...

Watson Labs Model 10. Auditioned in latter 1970's. A visually striking very large speaker. Tweeter and mids set in a separate enclosure atop a large bass unit with two drivers.

Sound: room filling, accurate, large sound stage, tight bass. WOW. Should have bought a pair, but $ too much for my budget. I may have dodged a bit of a bullett though as the tweeter/mid-section had some gas filled chambers that over time likely would have leaked. Oh well. I was happy with Dan D'Agostino's satellite/sub speakers. First ever in the US at the time. Still have them, but they take a backseat to my 1986 Thiel CS2's.