Dude this looks so cool, thanks for the recommendation

Taxi Driver is the least consistent movie I have seen

Try:

The Shining (1980)

Here's something famous for doing just that:

Robert Altman's neo-noir film The Long Goodbye.

Every single piece of music in the entire film (except for the End Titles' Music, which features a satiric use of Hooray for Hollywood) is a variation of the same piece of music: "The Long Goodbye" --composed by John Williams! (It's one of his most unique scores.)

But the thing is, each time it's heard (mostly diegetically) it's re-composed for an entirely different genre, in a way which is in tune with the scene: There's a Blue's Version, a Piano Bar Version, a Jazz version, a Mariachi Band version, composed as music for a Funeral procession; it's hummed by a thug, heard as Muzak in a grocery story, and used in many other ways. The opening notes of its melody are even used as the doorbell chime at the home of the movie's femme fatale.

It's mildly obscure but if you can overcome the one inch barrier then the Italian movie Leolo has Tom Waits' Cold Cold Ground playing throughout and is a decent film.

That's from Badlands

Jail Bait from Ed Wood has this guitar playing in the background practically through the entire movie

Not a movie but Twin Peaks had a constant loop

Back to the future!!!

Nice, now I can torture myself.

“Twisted Nerve” by Bernard Hermann. Soundtrack for the film of the same name, released 1968.

Are you talking about the movie's score or a song that's played in the movie? There's literally hundreds of movies with great scores. Some iconic scores would be The Godfather, Star Wars, Raiders of the Lost Ark, The Good, The Bad and The Ugly, Once Upon a Time in the West, Blade Runner, and Lawrence of Arabia.

If you're looking for examples of movies with specific songs, then I would suggest Fritz Lang's M, Casablanca, pretty much any Quentin Tarantino movie but more specifically Jackie Brown, Pulp Fiction and Death Proof, 2001: A Space Odyssey, A Clockwork Orange, Goodfellas. There are a lot more, but these are what I can think of right now.

The whistle song from Kill Bill 2 used to get stuck in my head for days.

True Romance and Badlands have a great little steel drum tune all throughout

Between Two Ferns has the same theme as Taxi Driver

Inception uses Édith Piaf’s “Non, Je Ne Regrette Rien” at varying speeds—depending on which level of dream you’re on.

The Card Counter

Beverly Hills Cop?

Qualen was preparing for contingencies

Live and Let Die used a feature of the theme song throughout the movie.

For some reason all I can think of is Home Alone

The Batman with that Nirvana song littered through out

True Romance with the xylophone song

The TV Tropes page for leitmotif in film.

Maybe not exactly what you're looking for, but a good starting point.

> I can only think of taxi driver

This is a thing for most other movies Bernard Herrmann did the score for too

The Long Goodbye's score is literally just one song played in different arrangements

Stalker and L'Intrus are a couple other movies I can think of with basically just one tune as the score, sometimes shifting a little bit or adding/subtracting

The Sting?