I would say Mozart has the easiest Sonata's and Beethoven Sonatas contain the most difficult. But comparing sonatas to etudes to Bach WTC? It's apples and oranges.
. . . agreed, except that I think Mozart is incredibly difficult because of the exposed nature of the writing. That said, his sonatas, for the most part, don't hold a candle to Beethoven's. Odd, isn't it? How uneven Mozart can be? The operas, the late symphonies, the concerti, the early and middle period chamber works are beyond comparison. But, he has these odd gaps, where inspiration lags and he sounds like some 18th-century journeyman. Well, I guess we can't parade our genius on every outing.