Vivaldi's detractors used to say he wrote the same concerto time and again so Williams is in good company.
On the (sort of) subject of Vivald, had the rules been different I would have nominated Max Richter's beautiful "On The Nature of Daylight" from Arrival which is one of my favourite films of recent years. Written prior to the film but it fits so well you could easily assume it was composed specifically for the scenes it so movingly accompanies.
Similarly though he hadn't even played it for donkeys years and was totally obscure, Anton Karas's theme for The Third Man is disqualified too sadly.
Of the big hitters no one has mentioned Michael Nyman yet. Though it's neither his best or his most successful I'm going to go with:
Chasing Sheep Is Best Left To Shepherds - Michael Nyman (from The Draughtsman's Contract)
Because...
It's a Peter Greenaway collaboration, a paring I've enjoyed over the years.
It shows the fuzzy nature of many soundtracks, though the soundtrack was 100% written for the film it deliberately reuses/riffs off elements of Purcell.
Most importantly it compliments the film brilliantly and sets a tone, aiding the narrative and another voice to that of the actors in the way that great film music should.