Piano Forum

It's a good question. The problem is that you probably won't get an answer here, at least not a complete one, because the Schubert sonatas are underplayed and you won't find individuals that have the required experience for a complete grading (if there is any "official" grading, I don't know). Having played some, but not all of them, I will try to rank the last 8 in terms of technical difficulty (from hardest to easiest).

1. Sonata c minor, D 958
2. Sonata D major, D 850
3. Sonata A major, D 959
4. Sonata B-flat major, D 960
5. Sonata a minor, D 845
6. Sonata G major, D 894
7. Sonata A major, D 664
8. Sonata a minor, D 784

Apart from the first two that can be set apart as very hard, differencences among the other sonatas are less important imo. For example, 7 and 8 are a close call, personally I find the last mvt of 664 very challenging to master well, more difficult than the last mvt of 784 which, even though virtuosistic, flows better.

PS: I am bewitched by Schubert's sonatas, after having tasted them, it is difficult for me to go back to Beethoven and like his sonatas as much.