Rachmaninoff is difficult to guage, in my opinion. Of course, he wrote a few monstrous pieces (Piano/Cello Sonata, 3rd/2nd concertos, Correli Variations, blah blah blah), but much of what sounds difficult is really quite easy after a bit of practice, and much of what sounds simple is unbelievably difficult (for me, anyway - I've had some fair amount of experience with Rachmaninoff).
I would say the same about Liszt, though - much of his writing is much easier than it sounds, although I guess Mazeppa or Feux Follets are as difficult (or moreso) than they sound.
Henrah, I'm not sure whether Chopin's or Liszt's etudes are more difficult - I've heard arguments for both, and as I've only played some of Chopin's etudes, I can't really say one way or the other.
As for Wieniawski, he was a violin composer - sort of like the Moszkowski of the violin, but I like Moszkowski's piano concerto, so I didn't want to use him as an example.

Henselt was a German (?) composer/pianist during the romantic era who lived a large portion of his life in Russia. Both of these composers wrote a couple nice pieces, but for the most part their music suffers from the left-hand-arpeggio-right-hand-melody syndrome, if that makes any sense.
Benchmarks are silly, but whatever, here's my list (adapted from yours):
Bach - WTC
Beethoven - Moonlight Sonata
Chopin - Etude Op. 25 No. 11 (Winterwind)
Liszt - Mazeppa
Ravel - Gaspard (Scarbo or Ondine - there are a few who find Ondine more difficult)
Alkan - Who cares?

Sorabji - Opus Crazystuff
Xenakis - Evryali