Schumann has 3 piano sonatas...
No. 1 in F#m, Op.11
No. 2 in Gm, Op.22
No. 3 in Fm, Op.14
(I just wonder why the 2nd and 3rd don't follow the chronology of their opus...)
Anyway, I never heard and played #3 but I have seen the score. I play #1 but I have chosen only the beautiful 3rd movement (Scherzo). Yes, #1, all in all, is the hardest.
I'm currently learning #2 and I'm done with the first 3 movements - just left with the Finale. Actually, this is difficult too but may actually be the easiest if to be comapred with #1 and #3. Also, the 1st movement is marked "So rasch wie moglich" or "As fast as possible." Argerich has a recording of this and it's awesome.