The don juan is considered the most difficult of his transcriptions - it's actually a paraphrase - and one of his most difficult pieces, period. The faust waltz is the only one i've played on this liszt. Not really that hard untl the last few pages. Oh no, wait, i've done the lucia, too. And if i can play it, anyone can. Requires delicacy - not bombastic octaves.
I'm currently working on the aida paraphrase which is also a delicate piece with some late lisztian harmonies and modulatuions. Again, not your typical show-stopper. I'm also doing the lohengrin festival and bridal song. Bombastic repeated octaves. Well, you've got to wake them up after you've put them to sleep.
Oh wait, i've done the liebestod, too. Not difficult and musically gratifying. If you're into wagner. There's that other edition they posted here a while ago which is more pianistic in many ways. I hate it when liszt uses those octave tremolos in the bass.
I'm also having fun with the ride of the walkyries. A tausig transcription.
The rigoletto is overdone, but also not that difficult.