What is the best sound system you ever heard?

In the old days ....a Decca cartridge (don't remember the arm or t/t), Audio Research electronics, and Infinity IRS speakers.

Here is a system I heard at a friend's house in 1978: Thorens TD125/Rabco SL8/Panasonic Strain Gauge cartridge, Marantz 7 preamp (used as a line stage), pair of Futterman H3AA OTL amps and a pair of Quad 57's. After hearing this I resolved to get a similar system!

Complete MBL system.  Radialstrahler 101, 9011 amp, 6010 pre-amp, VPI Aries. 

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. 

Nothing is more absurd than to compare recorded music to live, yet people do it all the time.

Easy for me to answer; this Magico/Solution system I described a few years back:

https://forum.audiogon.com/discussions/2c-hifi-epiphany-worlds-best-system

Nothing has come remotely close for me. Returning to the realms of reality, this is my current 2C analog/digital system that sounds and looks fantastic to me:

https://systems.audiogon.com/systems/7324

@orpheus10, but even after a hundred years or so that's one of the hallmarks, a key reference point of all audio (domestic, film, broadcast etc), isn't it?

Of course you can't reproduce the  acoustics of the Albert Hall in your car but you could still get an idea, couldn't you?

One sound system comes immediately to my mind and is open to the public (with some advanced planning).  Synergistic Research's listening room in Santa Anna, California. Every aspect of what can affect sound has been considered and addressed. Also, it is one of the few rooms where Digital, Vinyl, and Tape are all optimized.

For me, it is a great learning experience to use this listening room as a benchmark for my systems.
David Pritchard

Not an electronic system, but the best sound I've ever heard was the Vienna Philharmonic Orchestra playing in the Musikverein  in Vienna. 

@elizabeth...Sadly, speaks to my failure to go anywhere.... My own.

Believe me, most of the systems set up during exhibitions will not outperform what you have carefully set up at home, components selected to your liking, possibly spent lots of time on room-tuning and so on. 

For me, its also my own system...before I moved house. (previous room was tuned during nearly 10 years of time, now have to start all over again). On the other hand, that's why I like this hobby....

It would certainly help one in developing a good sounding system to be able to listen to other systems, whether they were better or worse. But especially if they were better, you know, to give one a target to aim for. Compared to what? Stovepiping has its drawbacks. Tennis players oft comment they improve their game by playing better players.