The best speaker you ever heard?

Easy. The Wisdom Audio Adrenaline Rush and M-75 systems. They have the speed and transparency of the best electrostatics (without beaming and the "hey I'm a hifi speaker" effect), very low and fast tight bass (to 12 Hz in my room), wonderful dynamics, best imaging I've heard from any speaker. They just disappear and play the music.

I've never heard a large system integrate everything in a natural way. They always seem to foul up the bass to my ears. Live bass is much faster than most of those big drivers produce it. I have no best but consider the following very memorable in their own way:

Apogee Stage and Scintilla (the most transparent speakers I have ever listened to)
Quad ESL-63- First time I heard them I was absolutely smitten. They still have a midrange to die for and if I were to have a 2nd system it would be based around them.
SoundLab A3 I haven't heard the big boys but I can imagine, wonderfully coherent as Albert notes but not too dynamic, damn what a shame because they do everything right, I do believe the big Soundlab might be "The last speaker you'll ever own".
IRS Beta's (except for that damn bass the rest was superb)

Merlin Millinium OK so I'm prejudiced because I own them, but I LOVE their coherence and their point source nature, I can't hear the drivers crossover and this is very important. They don't have wall shaking bass but what there is is very clean, quick and natural. They fill up a room with music without bringing attention to themselves, they just disappear. They are tonally neutral and have a liveness and an immediacy that I'll never tire of. Lastly, they have a size that is easy to live with. I would qualify them as without a shadow of a doubt the best two-way speaker I ever heard, sorry Wilson.

How about the Jean Marie Reynaud Twin MK II at $845.00 ?

Pipedreams. Uncannily capable of recreating an acoustical event as it was recorded in its original space. I guess that means a sense of effortless scale. Their ridiculous size precludes home use for me, but I really felt like I was in Jordan Hall (in which I listen to live music at least monthly) when hearing Zander's BPO playing Stravinsky's Rite in that infamous live recording on Pickwick (1990). Spooky stage realism....and NO bass overhang!

Jordan Hall. you're making me drool! I'm stuck half a world away, ahhhh to be there