he was the only other melancholy composer/pianist since chopin
I'm pretty sure that's not true. Edward MacDowell would fit that description, just for one. Perhaps Scriabin also. Or Anton Rubinstein. And one could go on . . . .
I find that PLAYING Rachmaninoff is in some ways more interesting than just listening to his music. Rach. has something special to offer to pianists, moreso than to just the run of the concert hall audience.
One important thing about Rach. for this forum is that he was predominantly a composer
for the piano.
It is hard to assess how "great" Rachmaninoff was. As for his influence on
me personally, he would be right up there in perhaps the top six. But as a pianist, I do not include many composers (example: Ralph Vaughan Williams) who really did not compose for piano.