The two songs in close competition for my favorite pieces of classical music are Handel's "Joy to the World" and Pachelbel's "Canon in D Major." I love Pachelbel's Canon so much that I actually own Pachelbel's Greatest Hit, a CD that has eight different versions of the song on it. I've also been listening to "Alleluia (To Pachelbel's Canon in D)" by Robert Gass & Wings of Song on repeat for the last couple of days. It's hard to choose a favorite version, but I do very much like this one by the Canadian Brass.
Canon & Gigue in D - Canon - Canadian Brass