For Messiaen, the Transfiguration of Christ and his opera St. Francis of Assisi are excellent. Unfortunately, as introductions, both are long works (90 mins and 300(!)mins respectively). They are both works of transcendental/esoteric Christianity: haunting mysticism in repeated leitmotifs and themes. I recommend both works. I have also enjoyed his organ music when heard live. I think I have the libretto to St Francis somewhere if anyone wants it scanned. I haven't heard the recorded version of the opera (only seen it live) but the best recording of the Transfiguration is the Orchestre Philharmonique de Radio France conducted by Myung-whun Chung.
On Amazon hereFaure: I only know his requiem, which is beautiful but perhaps slightly saccharine when compared with the Brahms requiem.
Messiaen and Charles Ives are both excellent, transcendentalist composers.