Far too many to pick just one.
Henryk Gorecki's Symphony no. 3 (Symphony of Sorrowful Songs) If you can, get the recording with Dawn Upshaw as the soloist.
[youtube]zKk-w_0SpSw[/youtube]
J.S. Bach Goldberg Variations (I like Glenn Gould's renditions)
[youtube]g7LWANJFHEs[/youtube]
Rachmaninoff Vespers - I actually heard this piece performed live in an old cathedral when I was a student in Leningrad (now St. Petersburg), so I got the full benefit of an entire section of real-deal Russian
bassi profundi.
In this one you can hear them go down to a solid low-C. Gives me the shivers.
[youtube]w3yAlequsRc[/youtube]
Du bist wie eine Blume, by Schumann is a beautiful little song. The words just kill me. It is perfectly beautiful in every respect.
[youtube]C_bZt63v0TE[/youtube]
Dietrich Fiscker-Dieskau singing anything, but especially Schubert, like Des Baches Wiegenlied.
http://www.listeningarts.com/music/art_song/oxy116/Music/20._Des_Baches_Wiegenlied.mp3
My absolute favorite of Schubert's Lieder is Am Meer from the Schwanengesang song cycle, though I don't like any of the variations I've found on Youtube. :/ Get your hands on the cd of Håkan Hagegård singing it and you won't regret it.
http://www.amazon.com/Schubert-Schw...ECDU/ref=sr_1_1?ie=UTF8&qid=1248805829&sr=8-1
Edit: Ok, I take it back. This isn't a bad clip, although I prefer a baritone singing it to a tenor.
[youtube]9lt5-0LojAM[/youtube]