Any Rach/Chopin/Debussy/Scriabin etudes that are under 2-3 minutes are usually ridiculous.
Scriabin's Op 65 #3 is pretty awesome sounding, but seems very awkward once I look at the sheets.
Virtually all of Rach's Op 39 can be used (save for #2, #5 and #7), and they have fiendishly difficult passages throughout. His Op 33 #6 in Eb minor is also one crazy piece of work.
Also, his prelude in Eb minor looks very similar to Feux Follets by Liszt, but is only 4 pages and about two minutes of double notes

Debussy's Octave, Chord, 4 fingers, and Chromatic etudes are pretty interesting. I don't know the numbers, but those are the techniques in each.
Some of Chopin's preludes too: #19 Eb major has difficult parts in every measure I'd say, along with #16 in Bb minor, and #24 in D minor. I'd stand by the argument that those preludes are among some that are harder than a good chunk of his etudes. The jumps in #19, the speed of #16, and the insane scales/thirds/arpeggios that come out of nowhere in #24 pose serious challenges.
I'm not a huge fan of them, but some Godowsky's Chopin Etudes are short enough, and are probably (I've never tried any) very difficult.
Fur Elise is a very hard song.
...did I just say that?