It would have to be a toss up between Moonlight Sonata and Mozart's Symphony 25 in G Minor; those are my two current favorites that I seem to always fall back to.
A few are coming in at a tie for me right now. In no particular order:
"Sleep" by Whitacre
"Allerseelen" by Strauss
"Angels in the Architecture" by Ticheli
"Vesuvius" also by Ticheli
"Jupiter" by Holst
"Concerning Hobbits" by... Raine I think? my copy on iTunes is a little confusing to find the arranger/composer.
and I've taking a fancy to Vocalise by Rachmaninoff .
Even though the person who played it was a little off on the notes, the actual rhythm has intrigued me.
Since you said classical, I will keep myself within those contraints of that time period. If it's that period, I would say Mozart's Sinfonia for Violin and Viola. It's just boss.