In no particular order except that I think the top 5 are among the worst of the worst.  More could be added I'm sure, but these are some of the most hair-ripping, ear-bleedingly bad ones.  I focused on original compositions and particular incarnations of well known songs that have become ubiquitous or just really stood out for their unique awfulness.  So, for example, there are any number of bad versions of every Christmas hymn, but Twisted Sister made the cut with "Oh Come, All Ye Faithful" for sheer audacity.

1. Last Christmas, Wham! (re-incarnated by Glee and Taylor Swift)
   
2. Wonderful Christmas Time, Paul McCartney
   
3. Santa Baby, Madonna (and many others of course)
   
4. Christmas Shoes, Newsong
   
5. Same Old Lang Syne, Dan Fogelberg
   
6. All I Want for Christmas Is You, Mariah Carey
   
7. Santa Claus is Comin’ To Town, as destroyed by The Boss (no disrespect Bruce)
   
8. Christmas in America, Kenny Rogers
   
9. Don’t Save it All for Christmas, Clay Aiken (Celine Dion's version was a close runner-up)
   
10. Grown-Up Christmas List, Amy Grant
    
11. Christmastime, The Smashing Pumpkins
    
12. Dominick the Donkey, Lou Monte
    
13. 8 Days of Christmas, Destiny’s Child
    
14. Merry Christmas, Happy Holidays, *NSYNC
    
15. Mistletoe, Justin Bieber
    
16. Oh Come All Ye Faithful, Twisted Sister (in the style of We’re Not Going to Take It)
    
17. Please Daddy (Don’t Get Drunk this Christmas), John Denver (sorry JD, but what were you thinking?)
    
18. Santa Claus Lane, Hilary Duff
    
19. This One’s for the Children, New Kids on the Block
    
20. You Make It Feel Like Christmas, Neil Diamond

There were at least three that didn’t make the list because they were just so wrong I didn’t even want to spread awareness of them.

Peter Leithart's new First Things article, "How the Church Lost Her Soundscape" is an absolute must read.  I've excerpted it below, but please go read the whole thing.

"The desire to make worship more appealing to young people was a major impulse behind the development of contemporary Christian music in the first place. The magnitude of this shift cannot be overestimated. Culture is a gift from the old to the young, and the younger generation’s grateful reception is a sign of honor for fathers. Cultural transmission has been thrown into reverse, also in the church."


"For all its variety, pop music is dismally monophonic. Transgression is encouraged, so long as it doesn’t get too close to the music. Lady Gaga wears her meat dresses and Rihanna feigns sex on stage, but when the music starts they are both as frothy as Justin Bieber. There can be no Stravinsky of pop music."


"Expertise is one of the values of modern culture, but expertise has always had a limited scope. We trust experts in physics and computer programming and perhaps foreign affairs. But the suggestion that there are experts in aesthetics, musicians who know what music one should appreciate, is greeted with hostility, also in the church. 'I know what I like' stops every argument, buttressed by 'Musical taste is subjective.' Lebanese organist Naji Hakim has lamented that in the Catholic Church 'many in positions of liturgical responsibility, with no musical education as regards technique or aesthetics, have come to believe in a tabula rasa, denying any lineage whatsoever.' Professional musicians have been 'sidelined' as 'the least common denominator has become the rule.' He wonders whether Catholics 'realize the level of mediocrity which the present liturgy has reached.'"


"The church created the soundscape for Western Christendom because she cultivated her own musical life in the liturgy that united human voices with the angelic choirs of heaven. I can hardly imagine a more worrisome sign of worldliness, or clearer evidence of the church’s identity crisis, than our eager renunciation of our own soundscape and our determination instead to reproduce the world’s."