Generally i can tell the difference between a good song, a great song, and a phenomenal song by the goosebumps i get. Now, the labels "good", "great", or "phenomenal" aren't set in stone; the difference of time can change how i feel about a song. The songs that follow, however, i have yet to demote from "phenomenal".
Lamentations of Jeremiah: i sang this with my high school choir at Legacy my junior year (so about six years ago). Granted, this YouTube video isn't us singing but it does the song the most justice out of the videos i could find.
Lindsey Stirling & Peter Hollens: The Skyrim Theme song. Peter Hollens sings ALL of the parts. Impressed? You should be.
Transformers: Arrival to Earth (by Steve Jablonsky)
The Da Vinci Code: Chevaliers de Sangreal (by Hans Zimmer and choir) The slow build in this is just amazing.
Harry Potter: Lily's Theme (by Alexandre Desplat) if this doesn't tear at your heartstrings going i don't know what will.
Lea Michele: O Holy Night. i don't care what time of year it is; this song is ALWAYS appropriate and ALWAYS phenomenal.