Is it okay to have some reservations about the question itself?
Don't get me wrong -- I like Rachmaninoff. However . . .
His melodies -- e.g., the concertos, Rhapsody on a Theme by Paganini, etc -- are rather saccharine and very sentimental. I'm not sure I would find him a great melodist. Although I'm not very familiar with that word.
It's appropriate that you compare him to pop composers, and the readiness of pop orchestras to transcribe his themes reflects the kind of "movie theme" aspect of his melodies.
I would say Ralph Vaughan Williams would be a good choice to answer this question, although he took many of his themes from English folk music and arranged them.
I think a melodist is a composer who can write a good "tune", and the great melodists are the composers who write tunes people remember, such as Mozart, Schubert, Chopin Liszt, Tchaikovsky, Brahms Schumann. Alzado your analogy to pop music is dead on, if a composer's pieces end up in commercial arrangements, (painful though it is)they are probably a great melodist, because the great majority of listeners (not wonks like us) respond primarily to melody. So do we, but we're distracted by subdominants and row inversions and developments and bitonality and 2nd subsidiary subjects and stuff like that.
But to answer the question; first Puccini comes to mind, but he is a contemporary (roughly) of Rachmaninov. McCartney is definitely up there, but he's not a composer he's a songwriter; composers build and constuct music, songwriters or "tunesmiths" like Sir Paul, Stevie Wonder, Richard Rodgers, Irving Berlin, Billy Joel, just write melodies and chords, which is an art of building or constructing too, but does not involve counterpoint, development, orchestration, or any complex use of form.
Nothing wrong with songwriting, I'm a songwriter first composer second. But I will restrict myself to composers in answering this.
George Gershwin
Topic: the greatest melodist since rachmaninov?