Hackers. The movie is great, the soundtrack is greater.

Interesting the divide here between who think soundtrack is orchestral music composed for the movie and others who think its a bunch of pop songs the director liked.

I haven't actually seen Tron: Legacy, but I assume the film cannot possibly be as good as the Daft Punk album it came with.

Gangsters paradise - Coolio

Movie: Beautiful Mind

Scott Pilgrim vs the World each track is shear awesomeness (...and the movie is not bad either).

Im not sure but I have seen all sorts of alright or crap movies broadcast and then the credits play and its like. wow this movie had this song!

Queen of the Damned was an abysmal movie with a surpassing solid soundtrack with some of the biggest names in music at the time collaborating.

Jonathan Davis of Korn wrote half the songs and sang in the movie, but his record contract stated that he couldn't appear on an album put out by another label, so the solution was to bring in guest artists to sing instead. Chester Bennington from Linkin Park, Wayne Static from Static-X, David Draiman from Disturbed, and Jay Gordon from Orgy. If you were a nuMetal fan back then, this was an all-star lineup.

Also, honorable mention to Batman & Robin for being absolute dogshit but having one of the Smashing Pumpkins' best songs on it.

The Phantom Menace

Spawn. The movie was laughably bad and I have banned it from my memory. However, the soundtrack is a collection of collaboration between metal bands and electronic artists and is really awesome.

Garden State

The Anna Taylor Joy Emma I though was great, and the soundtrack is amazing, but my wife only likes the soundtrack and not the movie.

Tron: Ares. The movie would be vastly improved if the dialogue track went missing.

Batman forever soundtrack is way better than the movie

Dazed and confused

This is not such an interesting question anymore because it's a trope to blow the budget on recognizable songs in a shit movie.

But if your looking for the biggest difference in quality, I'd say Singles (1992) Not that the movie is bad, just that the soundtrack is legendary.

EDIT: also because the best tracks are original recordings and not just a playlist of already popular songs.

American Beauty

To copy felsiq's comment, Sinners. The movie is phenomenal but the soundtrack? Holy moly.

Pacific Rim (film series)
Harry Potter (film series)
Ahsoka (television series)
LotR: The Rings of Power (television series)
The Hobbit (film series)
Moana 2
Frozen 2
Arrival (2016)

The last temptation of Christ, by Martin Scorsese with Willem Defoe. Soundtrack by Peter Gabriel. The movie is good, the soundtrack "Passion" is way better.

https://www.youtube.com/watch?v=IiJoTT6NDqU

https://petergabriel.bandcamp.com/album/passion-music-for-the-last-temptation-of-christ

[off topic]

The Curtis Mayfield album for the movie 'Super Fly' made more money than the actual movie.