I second the Prokofiev Visions Fugitives. I've worked on 1, 4, 8, 10, and 14, so I can't say much about the others (though I'm about to start 19 and 20). If you like fast, difficult pieces, I would especially recommend 14, and no. 8 for a slow one. You may also consider trying something from his set of pieces op. 12 (I think the most popular are the Prelude and the Scherzo). For Rachmaninoff, maybe one of his preludes (op. 23 or op. 32)? The popular ones are op. 23 nos. 4-7 and op. 32 no. 10 and 12. These are all very beautiful, but you may want to try something different. My personal "underplayed" favorite at the moment is op. 32 no. 3. Beethoven's 7th sonata is fairly difficult, so if you're comfortable with that, it opens up a fair amount of choice to you, though if you're not used to these composers' idioms, it may be good to start with something short. Another composer you may like is Scriabin, maybe not something late, but his preludes op. 11 or poemes op. 32 are a good place to start.