I'll chip in a bit for Haydn... Since everyone else seems to be leaning towards the heavier, beethoven type movements, check out Haydn's sonata in Eb HobXVI:49. It's similar to Beethoven's Op2/3 slow movement; calm first section and intense hand-crossing middle section.
For an easy slow movement by Haydn, the first sonata I learned, HobXVI:8 in G - Andante. Simple as they come, but the melody is so sweet, it's the kind that gets stuck in my head all the time. HobXVI:1 - Adagio is also a very simple, addictive melody. Advanced players could sightread these pieces.
A more intense slow movement, HobXVI:37 - Largo sostenuto. Some nice thick rolling chords, almost baroque sounding. Intermediate difficulty.
One of my favorite slow movements of Haydn is HobXVI:46 - Adagio. Calm, peaceful and joyous. Quite polyphonic, looks fairly difficult.