I listened to the Rondo (4th mvt) of Sonata no. 2 (op. 2) for the first time and was absolutely charmed. Sounds very much like a young Beethoven but is simply delightful.
Also the Rondo of Sonata no. 11 (op. 22) is very interesting; the middle part sounded kind of fugal (a hint of Hammerklavier, perhaps?)
The Andante Favori is as Schubertian as it gets.
Any others?
My favorite movement from number 2 is the first, allegro vivace. Hot damn, how those arpeggios build in the development, delicious!
Sonata 6, op10 no2, allegretto. The way the initial theme comes rumbling from the bass, sounds like this movement's come crawling out of somewhere dark. The next movement is a lot of fun too. Playful and bouncy, reminds me of being a kid.
I love finding those great but less known pieces. For some reason you've either never heard them before, or heard a few times but hadn't quite noticed before. They're all over the place really, there's so dam much good music that people wouldn't have heard before! especially in the vaults of the great composers, Haydn is always surprising me.