I wondered, what Haydn Sonatas would you recommend to study, that is, which of his many sonatas really worth their while.
My two favorites of the Haydn sonatas are Nos. 50 in C (as Hunwoo suggested) and 62 in E-flat. The most recent Haydn sonata I learned was the E-flat and it is one of the more virtuosic sonatas of his (as virtuosic as Haydn gets, anyway). It is also very beautiful, with lovely melodies intertwining themselves in all three movements.
A thorough study of either of these sonatas would definitely help you improve musically and technically.