Well, if all you're looking for is a difficult composer, there are certainly plenty out there. Godowsky, Alkan, and Henselt are in my opinion the most difficult tonal Romantic composers, roughly in that order.
If you go to the twentieth century, there is a vast pool of composers that difficult that their music borders on the physically impossible-or sometimes might just be impossible. Here are a few well known names:
Sorabji
Xenakis
Ligeti
Boulez
Finnissey
Feinberg
and there are many more lesser knowns
Of course, there are the standard composers which can still be very difficult-Ravel, Bartok, Prokofiev, Ives, Barber, Busoni, Brahms, etc.