Mendelssohn Variation Serieuses
A little more difficult:Beethoven: Variations in G major on the aria "Nel cor più non mi sento"Mozart: variation on "twinkle little star"Schumann ABEGG would be here.
Reger-Bach? Surely those are the best. And probably a bit easier than Soraji, Rzewski, Rachmaninoff and Godowsky.
Most of the variations you have listed here are among the most difficult virtuoso works you can play:Brahms paganiniAlkan Festin d'esopeRzewski's "The People United Will Never Be Defeated"
? ? ? The Abegg Variations are much more difficult than either the Beethoven or Mozart.