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so far we have seen:
-early Pag studies
-Grandes etudes (early TEs)
-a bunch of transcriptions: Beethoven Symphonies, Don Juan, Tannhauser, William Tell King Lear
-TE 4 and 5 (Mazeppa and Feux Follets)
-Mephisto waltz 1
-St-François de Paule marchant sur les flots
-Funérailles
-Totentanz (solo version and piano/orchestra)
-Grand galop chromatique
-Vallée d'Obermann
considering mechanical aspect mostly, let's take away a few:
Funérailles, Vallée d'Obermann and "St-François de Paule marchant sur les flots" are no beasts in the Liszt catalogue. They are actually good starting points when it comes to Liszt. Most of Mephisto is not hard, but there is one very nasty part and yes, it is possibly the most butchered Liszt piece. I personally don't think it is reason enough to put it on a "hardest Liszt list". If butchering ratio was to be considered, the hardest pieces ever would be Chopin's fantaisie-impromptu and Beethoven's Moonlight sonata.

Grand Galop is not that bad, Cziffra speed is just insane. Not many can play it at that speed. So if we do keep the insane speed restriction, yes, it's extremely hard. Otherwise nope.
For the transcriptions I will stick with the mainstream stuff: Tannhauser, Don Juan and the Beethoven symphonies are BEASTS. A lot of mostly unknown transcriptions are also incredibly hard.
Concerning TEs, 4-5 and 12 are recognized as the hardest. We don't seem to agree on which is hardest. I would personally say 5 is hardest. I did play 4 and find tremolos quite easy so I am not scared at all of #12. Someone asked how hard is # 8 Wilde Jagd? It's much easier than it sounds, unless we put the restriction under 4:30...
That being said, the early TEs (Grandes études) and the early Pag studies are much harder than the TEs overall. I find these pieces less effective than their final product so I don't really see the point of actually learning them seriously but they are insanely hard and it is the point of the current topic.
I never seriously looked at Totentanz so I won't pretend I know all about it's difficulty, but I did know some student that did play solo and piano/orchestra versions. He actually said it was not that hard and quite fun.