Tron Legacy, but that's cheating as it's essentially a Daft Punk music video.

The Matrix
O Brother Where Art Thou?
Forest Gump

• Pirates of the Caribbean (personally, At World's End has the best, Hans Zimmer)
• The Lord of the Rings (Howard Shore)
• Gravity (Steven Price)
• Tron Legacy (Daft Punk)
• Moonlight (Nicholas Britell)
• Harry Potter (can only speak to the ones by John Williams)
• Braveheart (James Horner)
• The Matrix (Don Davis)

More general answer than you expected, but...

I really like when the sound design in movies makes sense - it "plays" in radio, gramophone...

For example "Black hole sun" cover in first episode of Westworld is just brilliant and caught me off guard (I didn't know what it is about).

Birdman. Just drums. Really fucking good drums. Also they appear in the middle of one scene in an excellent way.

Also: not a movie, but Cowboy Bebop. Lots and lots of great tracks.

Once upon a time the west. (Ennio Morricone)

Blade runner (Vangelis)

The Lord of the Rings

This is Howard Shore's Magnum opus. It's what distinguishes this movie as more than just a great adaptation. His use of themes to represent not only races and kingdoms but characters, objects (like the One Ring, of course), and even concepts is a level above most movie soundtracks. There are even elements of storytelling through the music!

For example, the first time we hear the theme for Gondor is when Boromir is in Rivendell. Since he's more or less alone, the theme is played by a single French Horn in a somber (almost tragic) style. In Return of the King, we see Minas Tirith, capital of Gondor, in all its glory, and so the full orchestra plays the theme.

One more: As the Fellowship begins to break down, so too does the theme. We go from heroic phrases to shorter, interrupted instances. There's a book about the soundtrack written by Doug Adams. I highly recommend it if you're interested!

The Blues Brothers - it's stringing together performances from famous musicians, and the soundtrack was successful as an album in its own right.

• Ray Charles
• James Brown
• Aretha Franklin
• Cab Galloway
• etc.

• Spawn
• Baby Driver
• Garden State
• the Lord of the Rings trilogy

I know you said movies, but a soundtrack for a show I'm hooked on currently is Legion (FX/Marvel, on Hulu). The whole entire show has an amazing cast to begin with, but Jeff Russo (Fargo, Star Trek Discovery, more) and Noah Hawley put together one hell of a score. I highly recommend it and the show.

Dazed and Confused

Can't believe no-one mentioned The Crow..

I am quite fond of the recent Dune movies' soundtrack. Hans Zimmer can make a good bwowwwum, and a helping of One-Woman-Wailing :tm: also helps

Aside for that I would get into movie musical territory. A much derided subgenre that I adore.

A few not listed:

• The Crow
• Empire Records
• Leaving Las Vegas
• Get Shorty
• Grosse Pointe Blank
• Pulp Fiction (listed elsewhere)

Reservoir Dogs, O Brother Where Art Though, Blues Brothers, From Dusk til Dawn,

I'm going to reframe this as who I think the best composers are:

Bernard Hermann, Vertigo, North by Northwest, Psycho, etc.

John Williams, E.T., Jurassic Park, Indiana Jones, Star Wars, etc.

Akira Ifukube, most of the Shōwa era Godzilla movies

Shiro Shigasu, Evangelion, Shin Godzilla

As far as new work, I'm partial to Scott Stafford on Ultraman Rising. It came out on Netflix this past June and I was really surprised how good that movie and soundtrack were.

• The Village

• O Brother Where Art Thou

• Amélie

• Pride and Prejudice

• Shutter Island

to name a few.

For me the criteria is: would the movie be very different with another soundtrack. The below offerings truly elevate their movies imo.

Tron Legacy

Blade Runner

Black Panther

I quite like the Interstellar soundtrack.

Akira(1988)

The soundtrack compliments the action so well.

I'm probably biased since this is my fave Pixar movie, but WALL-E, I love Eve's theme the most it's so beautiful and has this futuristic and hopeful kind of vibe to it that I really love

28 Days Later

Trainspotting

Garden State....could not tell you a single thing about the film but when it was released, the soundtrack was full of my favorite artists at the time

Honorable mention: Most Wes Anderson films have pretty thoughtful soundtracks but I've never been blown away or introduced to a new band

Watched Blade Runner 2049 2 days ago. Had a very good soundtrack but I wouldnt just listen to it just.

Personal favorites:
Tron: Legacy
Pacific Rim

The Fountain, Requiem for a Dream, or anything by Clint Mansell

The Big Lebowski deserves a mention.

The Matrix soundtrack is amazing

The Life Aquatic With Steve Zissou

Romeo and Juliet (the one with Claire Danes and Leo DiCaprio)

Pulp Fiction

Buena Vista Social Club

Help! (Or any of the Beatles movies)

Purple Rain

La Boum (French movie, 1980)

Fame and The Kids from Fame (the 1970's Original, not he reboot)

Blues Brothers

The Third Man has that fantastic zither theme.

A Knight's Tale

Eraserhead, Side B

The album has been seen as presaging the dark ambient music genre, and its presentation of background noise and non-musical cues has been described by Pitchfork's Mark Richardson as "a sound track (two words) in the literal sense". -wikipedia

The mood and tone of Eraserhead and its soundtrack were influenced by Philadelphia's post-industrial history. Lynch lived in the city while studying painting at the Pennsylvania Academy of Fine Arts, and was fascinated by its feeling of constant danger; describing it both as a "sick, twisted, violent, fear ridden, decaying place" and "beautiful, if you see it the right way."[8][9][1] Lynch and Splet used avant-garde approaches to recording on the soundtrack; including crafting almost every sound in the soundtrack from scratch using bizarre methods. The ambiance of the love scene in the movie, for example, was produced by recording air blown through a microphone as it sat inside a bottle floating in a bathtub.[10] Lynch and Splet worked "9 hours a day for 63 days" to produce the soundtrack and all of the sound effects in the film. Splet recalls the sound effects Lynch called on him to produce for Eraserhead as "snapping, humming, buzzing, banging, like lightning, shrieking, squealing” over the five years it took to produce the film and its soundtrack. -wikipedia

Good Will Hunting has a fantastic soundtrack.

Stranger Than Fiction, soundtrack by Spoon