A few candidates:
Verdi - Otello & Falstaff
Brahms - 4th Symphony
Mahler - Das Lied von der Erde; 9th & 10th Symphonies
Debussy - late sonatas
Janacek - all his late works
Strauss - Four Last Songs
Vaughan Williams - Hodie, Oboe Concerto, 8th, 9th Symphonies, etc.
Britten - "A Time There Was"
Shostakovich - 15th Symphony
Jim