Question

Asalam aleykum

Off all the 20 riwayats which one is the most difficult of all, jazakumulahu kheir.

Answer

Wa alaikum assalaam wa rahmatu Allah.

The answer would most likely vary from individual to individual depending on many circumstances and what they have been exposed to.  For some, the riwaayah of Warsh is difficult because it is so different from Hafs, but if one was exposed to Warsh all their life, they would not find it difficult.  The recitation of Hamzah with its two riwaayah has some difficulty in stopping on a word with that has a hamzah in the middle or end, but if one has studied the rules, it becomes less difficult with time.  The idghaamaat al-kabeer that the riwaayah of As-Soosiyy has is also different and requires knowledge of the rules, but is not difficult once one studies the rules and understands how to apply them.  

The easiest for those used to the riwaayah of Hafs would be Qaloon, Ibn Katheer, and Ibn Thakwaan.  

In the end, it depends on many factors.  May Allah grant you the knowledge of the qira’aat.  Wa iyaaakum.