What is the best sound system you ever heard?

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

Sadly, speaks to my failure to go anywhere.... My own.                                       
The only time I can say I heard better was in 1982.. Went to listen to speakers I wanted, in Chicago. Infinity RSIIa, was power by top of line Onkyo Grand Integra.. Listening to Carmina Burana, Orff.. The fact I could pinpoint each singer as a separate voice in the choir was the clincher for my buying the speakers.

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.