The Velocity of Saul at the Time of His Conversion by Okkervil River. Once, I got dumped and listened to it 72 times in a row. To be honest though, it was mostly on in the background as I cried.
One of my favorites too, Brittany. On a par with Westfall and Black Sheep Boy, and if I were to pick an Okkervil River break-up song, it would definitely be something off of "Don't Fall In Love With Everyone You See".
I think anyone who loves music would find it difficult to narrow down a favorite -- most music junkies I know have different favorites for different moods, moments, what have you.
Among mine in the rock/pop idiom:
Joanna Newsom -- Only Skin
Yo La Tengo -- Black Flowers
Okkervil River -- Westfall (the live version off the Overboard & Down EP)
Dresden Dolls -- Mandy Goes to Med School
Metric -- Raw Sugar
mclusky -- Forget About Him I'm Mint
Sufjan Stevens -- Chicago
Laura Veirs -- The Cloud Room
RJD2 -- The Horror
DJ Shadow -- Organ Donor (Extended Overhaul)
The Avalanches -- Frontier Psychiatrist
Spoon -- Jonothan Fisk
Built to Spill -- Twin Falls
The Decemberists -- The Legionnaire's Lament
The Unicorns -- Tuff Ghost
Elliot Smith -- Baby Britain
. . . .
etc.