Well, this is my selection for the top 25 musical numbers, lip synched or actually sung in a movie by one or more characters. You won't see Chicago or The Blues Brothers here. This is strictly things that occurred either off the cuff or were not part of the normal progression of the movie.
UPDATE I screwed up the numbering and omitted one of the greatest moments so I had to remove Clueless since it was number 20 and sometimes, there are losers. Like me! So, here is the redux list.Here we go...
25. The Jerk "Picking Out a Thermos For You"
24. Jerry Maguire "Free Falling"
23. The Replacements "I Will Survive"
22. Mr. Deeds "Space Oddity"
21. Tommy Boy "Superstar", "Come On Eileen", and "It's the End of the World as We Know It (And I Feel Fine)"
20. Sixteen Candles "Birthday"
19. Elf "Baby, It's Cold Outside"
18. Austin Powers 2: The Spy Who Shagged Me "Chili's Baby Back Ribs"
17. Ghostbusters "Santa Claus Is Coming To Town"
16. The Hangover "In the Air Tonight"