Definitely Farewell to Music. It was the first harp piece I worked on that I picked myself, not my teacher. She wasn’t familiar with it but once she heard it she loved it too.
My Second favorite is Robert Jordan, followed third by Fanny Power.
But wait, actually, it’s probably “Carolan’s Favorite Jig”… since it’s been my favorite tune to play for many years…and also one more slip jig I think it is, though I can’t recall the name..