Definitely Mazeppa! It's not actually very hard, and it will impress people (especially pianists!) no end.
A tip. There are four version of this in existence- the early (easiest) the second (from the hard version of the Transcendental Etudes), another version in between the two (my favorite one), and the version which everyone plays.
When I play this, I sometimes take bits from the other version (not the ultra-easy one, of course), to suit my own strengths and disposition. Sometimes, the earlier versions are more effective.
Another- ditch the consecutive 4/2 fingering for the thirds- use regular, legato third finger, and you'll be able to play it way faster.