Danny Elfman's Music for a Darkened Theater Vol 1 is a good compilation, although I may be a bit nostalgic with that call (it was one of my first score releases, along with Jurassic Park, on audio cassette).
Got to be Great Western Film Themes double album compiled by Alan Warner in the 70s on the United Artists label. Got me into Morricone and all the greats, and quickly followed by Western Themes Italian Style, same compiler this time devoted to Morricone.
The Phase 4 Herrmann albums. It was buying these for the Harryhausen music that turned me onto the rest of Herrmann's work.
Then there's the Polydor Miklos Rozsa Composer Conducts series. The one I still have has Thief of Baghdad, Knights of the Round Table, and Naked City on it.
And of course the Charles Gerhardt RCA series. My first was the Errol Flynn collection.
All in all a great start for a kid in his early to mid-teens.
Though it was never reissued on C.D., my favorite would have to be the United Artists 2 L.P. set "Ten Golden Years", which had the instrumental version of John Barry and Leslie Bricusse's "You Only Live Twice" (which was later put on the expanded reissue of "You Only Live Twice" under the title "Twice Is The Only Way To Live") and an alternate version of Barry's main theme from "The Knack" that was never on the C.D..
I'm surprised it's taken this long for somebody to mention the Classic Film Score series, surely the cornerstone of the film music revival that began in the 1970s.
This is probably tilting at windmills, but I would argue for more precise terminology. We could profitably distinguish between "collections" (or anthologies) that simply assemble a variety of diverse musical material and true "compilations." The Merriam Webster definition of the latter describes "a book or document composed of materials gathered from other books or documents." By this logic the original Gerhardt albums were collections, but the later "Spectacular World of Classic Film Scores" was a compilation, i.e., it drew materials from the existing albums. Similarly, John Williams's original Boston Pops albums of his own music were collections, whereas some of Silva Screen's second-generation repackagings were compilations. I'm not saying that the casual use of "compilation" is wrong. Lots of people use the term broadly. But precision would be nice.