That depends on your definition.
There are soundtracks that appeal to the masses, soundtracks that are a compilation of old songs (but compiled well), soundtracks with several hit songs on them, and soundtracks that have instrumental music (or sometimes music in general) that literally helps make the movie what it is. The feel, the excitement...the terror...it simply elevates the movie to an entirely new level.
If you simply look at the list of the top selling soundtracks of all time, a few standout to me (Wiki link :
https://en.wikipedia.org/wiki/Soundtrack_album). Saturday Night Fever had music that defined an era and was probably at least half of the movie. Purple Rain was essentially a Prince vehicle and unique in terms of both popularity and that the star was an actual recording artist who controlled EVERYTHING about his music. The Sound Of Music is obviously iconic and used music as a way to help tell the story. For me those are the 3 (along with Grease) that outshine the others.
Personally, when I think of great soundtracks, I prefer those that really enhance the feel, vibe, and mood of the movie and it's individual scenes. One of my personal favorites is the soundtrack from Donnie Darko, which combined original theme pieces and compiled tracks to form an excellent backbone that enhanced each individual scene. Under the Milky Way, Killing Moon, and Head Over Heels (loved the cinematography of the school montage) were excellently deployed and Gary Jules' take on Mad World is one of the all-time great covers, exceeding the original in every way, shape, and form.
Often the greatest scores are those that are purely instrumental. Star Wars has iconic theme pieces that are masterfully used. Aliens deployed a very frenetic score than enhanced the feeling of threat and fear while watching the movie. Obviously Jaws is another movie that employed iconic instrumental pieces to impost fear. It's been several years since I've seen it, but I recall the movie Zelary having a beautifully written score.
