There is no such thing as an easy classical piece, especially on the guitar. But here are some that are relatively easy. Please suggest others via a comment.
JS Bach: Prelude and Fugue in C major, BWV 846
From The Well-Tempered Clavier, this is easily one of the most beautiful pieces ever composed, purely for its simplicity and elegance.
JS Bach: Air, BWV 1068
Beethoven: Moonlight Sonata (1st movement)
Satie: Gymnopédie No. 1
Satie: Gnossienne No. 1