What are some of the most difficult piano works which only have the right arm? concerti, solo works.
I don't think in terms of more or most "difficult'. Either you like it, or you don't, and if you do, then you do whatever is necessary to master it. You may want to try Alkan's "Introduction, Variations et Finale", Op 76, No. 2. There are several clips available on YouTube.Alkan - 3 Grandes Études, Op.76 - score