Beethoven's 9th for me as well, followed by his 5th.
Bach's Air , too.
Imo the best opera aria is
Nessun Dorma by Puccini.
When it comes to soundtracks it are the
Theme of Once Upon a Time in the West by Ennio Morricone and
Summer Overture by Clint Mansell from the Requiem for a Dream Soundtrack.