If you want a prelude challenge, try Prelude 16, I'm working on it myself. Start with the left hand. Stay away from 19. Whatever you do, don't play 19. I cannot even dream of playing it after playing with the score for about 15 minutes. 
24 is very, very challenging. 12 was harder (for me) than most people find it. I didn't even get through it so I'm certainly not performing it. 8 is more manageable but still very very hard. I might recommend starting with prelude 3 as it is one of the few transition pieces from hard to very hard. Also consider 5 or 10 for more transition pieces.
BTW I'm playing 1, 3, 4, 7, 11, 15 and 16 so I can really only speak with certianty for those, but I would recommend any of these pieces.
If you want etudes, start with 10/6, it's the easiest for most people. 25/7 is not as challenging technically but musically it can be quite demanding and make you sound like a beginner. 10/12 is not as bad as it sounds and is great for left hand finger work... obviously. 
25/1 is manageable. 10/5 is not as "easy" as everyone says it is. Do not attempt 25/3, 10/3, 10/10, 25/8 or 25/9 until you have played some others as these sound doable but are very very difficult. 10/10 is very very very hard, and 25/3 and 25/8 are very hard, though they don't sound that hard at first. 25/9 you could probably play after you have a few etudes under your belt.
Just my 2¢. Good luck.
28/8, 28/12, 28/16, 28/19, and 28/24, IMO, are the hardest preludes. 28/12 and 28/24 are often disregarded as difficult preludes and only 28/8, 28/16, and 28/19 are considered difficult preludes. IMO, I think 28/16 and 28/24 are the hardest. The main difficulty of 28/16 are actually the leaps in the left hand which stretch up to two octaves in eight notes. You could find the right hand in any Mozart or Beethoven sonata. 28/24 has a left hand where even Rachminoff has to leap, and the right hand is just random stuff where at its peak, has a descending thirds chromatic-scale where you would not even find in 25/6. 28/8, 28/12 and 28/19 are easier but still very hard.
You should actually start with the Etude in f minor, composed in 1839. 10/6 is actually very difficult because everyone plays the tempo wrong. The sixteenth notes are supposed to be on 104.5 BPM, and everyone plays it almost half the speed, if not, even more. 10/3 is a beautiful piece, but the middle section is fairly difficult. Frankly, I don't know why everyone speeds it up to almost two times the speed; I don't see any piu mosso or accelendaro or that. 10/5 is actually very difficult, as pencilart said. Don't play 25/6 until you've mastered Czerny everything because his fingerings work very well in this etude.
10/1, 25/11, and 25/12 are actually among the hardest etudes, IMO. Lots of people (like me), at first glance, think that 10/1 is very easy. However, each set of sixteenth notes stretch octaves or tenths, which is like impossible to stretch unless you have huge hands like me. 25/11 reminds me of Hadyn. Once the Lento section is done, it turns into one of the hardest compositions ever written. For awhile, it's just right hand, but then the left hand kicks in and it becomes even harder. 25/12 is the easiest of the three, but it's still very difficult. I think you should stick to the easier etudes.
If you want a Beethoven sonata, go for the calmer (I never said calm; I said calmer) ones. You could try Op.26 or Op.78, or you could go for the harder Op.110. They're the calmer sonatas with slow first movements and aren't really Beethoven-like until the fourth movement; in Op.78's case, second. Op.78, btw, is in F# Major, and I don't know if you can handle six sharps. Op.26 and Op.110 have few parts in ab minor, too.
Topic: New repertoire!