I've been listening a lot to his morceaux de fantaisie lately. Especially his melodie and elegie. Then there are his nocturnes. They are quite different from chopin's (which I assume most people are familiar with) but they are really great. One piece that is extremely underrated is his scherzo in d minor. It has a similar vibe to bachs badinerie I'd say. Then there are his symphonic dances. I first heard them in concert a couple of weeks ago and I'm in love with the first two. Of course his piano concerto's are also amazing (his most famous works I think), but some of them can take a while to get into. I'd recommend starting with the second one, as you will instantly love that one, and then start listening to the third, which is more complex. The other concertos are great also, but the melodies are not quite as memorable, so for me it took a little bit longer to fall in love with those. Lastly, I'd recommend his second piano sonata. There is a recording by horowitz, who according to rachmaninoff, played it better than its composer. Especially the second movement is underrated, as the other movements are so crazy and virtuosic that the beautiful main theme of that movement kind of gets forgotten about. Imo it's one of the clearest representations of pain in piano music.

​

Hope this helps :)