Bizet's
Temple of the Pearl Fishers duet has to be in there. The interweaving of the two voices and the haunting melody raises the hairs on the back of my neck every time.
While we're in an Opera type mode... Turandot's
Nessun Dorma, but only by the one man who has really done it justice with his absolutely effortless rendition - Pavarotti. RIP Luciano.
Also, from the great film The Shawshank Redemption (if you haven't seen it, why not???) the following clip of the
S'ull Aria from Mozart's Marriage of Figaro (and
here without the film) - outstanding stuff!
Can't think of any non-classical/opera stuff at the moment.