I used to help out with weddings at my church. Help run the sound, play music, etc… and I have been to 100s of weddings. Anyone who has been to many weddings knows that the default processional is Pachelbel's Canon in D. In fact, when I was suggesting music for the wedding, and couples had no clue what to play, it was my first suggestion. It's popularity made it a nice, safe recommendation.
The downside, of course, is that the song really sticks in your head, just like this guy points out in his rant…