What is the best sound system you ever heard?

The Infinity IRS V speakers with $100,000 of associated PS Audio gear at PS Audio is a strong candidate for the best system I’ve ever heard. 

The one way up on Red Mountain Aspen CO, the one Dave Brubeck used for practice late 60s early 70s. All McIntosh everything. Some unknown monster midrange horn, time delay system, and concert grand piano of course. Also some kind of Joshua Light Show panels. I recall there was a big bunch of Ducati racing motorcycles in the garage.

Alchemist The Pre & The Mono (x 2) amplification, Alchemist Forseti CD transport & DAC, Nottingham Analogue Mentor turntable, Ruark Excalibur speakers.

That was 20 or so years ago of course, so how it would sound today I don’t know. Still, nothing I’ve heard since has come close to making the same impression.

A few other stand out impressions are:

KEF Reference 207 / Karan Acoustics KA I 180.

B&W Nautilus / Electrocompaniet Nemo mono blocks.

Avantgarde Acoustics Trio / EAR V12

In recent memory it has to the Kerr Acoustic K320 speakers. Nothing spectacular, but nothing obviously wrong - no sibilance, no bleaching, no cabinet issues. Eminently listenable fuss free sound through a Pioneer CD and smartphone streaming no matter what they were asked to play. Not sure about the amp, but it was a large black box with handles on the front.

I should also mention being stunned by the spooky imaging and scary dynamics of the Avantgarde Trios some 15 years ago. System featured a (ridiculously) deep plattered turntable with a, wait for it, £5k cartridge! Amps were some exotic valve types.

It was almost a time machine experience, but the trouble was the young chap demonstrating only played some 1950’s Jazz and didn’t seem keen on trying anything else. I still recall him telling us that it cost £5 in stylus wear every time he played a record.

Having heard systems of price ranges going well into 6 figures, got to say it's still a remote facsimile of live. Moving beyond that, I think my system is pretty excellent.