I'm not a huge fan of musicals -- although there are exceptions -- but I do find that a good musical performance can add a lot to a narrative feature: They can serve great comedic, satiric, rousing, or romantic value. In many cases, they also showcase the singing talents of actors we don't otherwise associate with singing. The history of film has seen some fantastic musical performances over the years, and the 15 below are my favorites.
Note that I've only included songs where the actors or actresses actually sing (or at least, in the case of Michael J. Fox, are supposed to be singing). Lip-syncing numbers, such as the car scene in Wayne's World do not count for the purposes of this list, although singing along with a song does.
Stranger than Fiction, "The Whole Wide World," by Will Ferrell
High Fidelity, "Let's Get It On," by Jack Black
Almost Famous, "Tiny Dancer," by the Cast.
Mulholland Drive, "Crying," by Rebekah Del Rio.
Before Sunset, "Let Me Sing You a Waltz," by Julie Delpy
Adventures in Babysitting, "Babysitting Blues" by Elisabeth Shue
Beautiful Girls "Sweet Caroline" by Cast.
Back to the Future, "Johnny B. Goode" by Michael J. Fox.
About a Boy, "Killing Me Softly" by Hugh Grant and Nicholas Hoult
The Meaning of Life, "Every Sperm is Sacred" by Monty Python.
This is Spinal Tap, "Big Bottoms" by Spinal Tap.
The Muppet Movie, "Rainbow Connection" by the Kermit the Frog.
Blazing Saddles, "Work Song" by Cast.
Magnolia, "Wise Up" by Aimee Mann and Cast.
Young Frankenstein, "Puttin' on the Ritz"