You’re not supposed to really, really like the Schubert-Rückert Lied Sei mir gegrüßt! because it’s considered repetitive and somewhat boring as a consequence. Schubert developed the song’s opening motive in his Fantasy for the Violin and the Piano in C-major, you’re told – and encouraged to enjoy that work.
Regardless, I’ve been rather fond of Sei mir gegrüßt! since I first heard it, two or three years back, but I did not know that it was Wagner’s favorite Schubert song. Ian Bostridge, one of the finest Lied singers of our time, discusses this and other Schubert pieces in a review of Graham Johnson’s “monumental three-volume encyclopedia of Schubert’s songs” in The New York Review of Books.
Even if you’re allergic to Bostridge as some are (not me obviously), take a look: his piece is illustrated with a photograph of Schubert at the Piano by Gustav Klimt, the 1899 painting that perished when retreating SS troops set the Immendorf Castle on fire in May 1945.