In my opinion, I think they are Rachmaninoff's most difficult set of solo-piano compositions (aside from the Piano Sonata # 2). The one I've seen I guess to be the most "manageable" since it has been in quite a few student sheet-music compilations is Op. 33. No. 8....it's the only one I've actually tried, and it's not so bad, except for the extremely agitated section in the middle; but it feels really pianistic since the notes fit into your hands quite well. And it's very beautiful.
But I know nothing of your technical proficiency, but my favorites, that I deem beautiful would be:
Op. 33, no 3 (this is actually used in the finale of the 2nd movement of his 4th concerto)
Op. 39, no 1 (has more of an awe factor rather than beauty....it's actually quite a terrifying piece)
and
Op 39, no 8 (the prettiest of the set, but a bind on the fingers....couldn't make it through the first line properly when SRing).
But when it comes to these pieces (like most of them) beauty is in the eye of the beholder. You should check them all out on youtube!