This is a hard question...my PhD specialty in the field of liszt can help with the three masterworks, and the only one of those i havent played is the barber sonata, so instead of helping where i know nothing, i will only rank the other six...i apologize that my repertoire doesnt cover the wonderful barber piece!
Easiest to hardest (easiest at top to hardest bottom):
Chopin Grand Polonaise (the depth doesnt compare to the other three, though technically speaking it IS intense)
Brahms Sonata No 3 (doesnt quite compare to his ballades, which are INSANE)
Liszt Mehisto Waltz (speed is really the only issue here, but adjusting to the waltz tempo and playing the piece as a dance and not as the insanely difficult filigree of asymmetrical chords and blisteringly paced action that is also so apparent)
Brahms Sonata No 1 (Yep, this is one tough piece, but again, for difficulty with Brahms, i would try the ballades or his variations on the paganini theme ubiquitous throughout romantic piano literature)
Sorry skeptopotamus, but i can probably find a few guilliard professors to side with me on this one:
Dante Sonata (i played this WAAAAY back in early college years and forgot it note-for-note (im trying to relearn it by the way) and its a wonderful piece. Technically, this is a monster, but not nearly as complete in the technical sense as its b minor counterpart. difficulty of ten is easily a 9+...however, its difficulty when compared to the b minor is almost insignificant)
The b minor sonata was my graduate degree piece (that and the tannhauser transcription) and it hits home that you asked about this. The notes alone are unbelievably difficult to memorize and retain. The technical difficulties are staggering and playing this piece with a flow and smoothness throughout while still capturing all of the nuances of tone and subtleties of phrase...this piece is almost unconquerable. The piece was DESIGNED to be mysterious, to be ineffably difficult. And I promise, If we're talking just notes this piece alone is hardest...but the second one mentions polish and playing this piece to competition standards and professional circuit rigor, this piece may be included in the hardest of all piano literature.
If you were looking to play one piece from this set, I would recommend the b minor sonata...it is beautiful and evil, balanced and unending. This is a masterwork in all of music...i would greatly encourage you to pick up the schirmer edition of this piece (or the Bolet memorial edition...he played it fabulously) and try the second movement first...it will amaze you how beautiful and lyrical this middle movement is. For show, the third movement is undeniably tense and fast, the first a wonderful creation of theme and counter-argument. This piece alone can introduce you to just how deep the piano can go in terms of sound quality and tonal sophistication.