Having recently contributed the following to the thread "The Greatest Opera Ever Written," I think I'll import it to this one in case there's anything in it of interest to anyone here.
Whether it's actually the greatest opera - with respect to everything that the complex art of opera can be - Tristan is, I think, the greatest single achievement in the entire history of music. There may be more perfect operas; but certain rare achievements in art are so immense, so stunning and awe-inspiring, as to make mere perfection seem irrelevant. I think immediately of Shakespeare and King Lear, of the late quartets of Beethoven, and of Wagner's Tristan und Isolde.
For its unimaginable expansion of the possibilities of the language of Western music, for its daring plunge deep into aspects of human experience no musical work had ever explored before, for its sheer intensity and visceral impact, this is the work above all others that, no matter how long and well we've known it - and perhaps all the more the better we know it - leaves us feeling that it could not possibly exist, that no human being could ever have dreamed of such a thing.
So much of what has happened since 1859, in music and even beyond music, has been what it is because of this singular work. Wagner may have equalled or surpassed it in one respect or another in subsequent works - the broad humanity of Meistersinger, the spiritual profundity of Parsifal - but when all is said and done it is Tristan which confronts us with an unaccountable eruption of genius without any parallel, which like a volcanic eruption changed the landscape of Western culture forever, for better or for worse.
Tristan went beyond anything even Wagner himself suspected opera, even music itself, could be. It astonished him even as he wrote it. If we have any idea of what we're hearing, it can hardly astonish us less.