Dean-
On your neck. Maybe it's because he was a vampire for a short time, or maybe it's because you always giggle and sctunch up your nose and close your eyes tightly.
Sam-
Cheek. He loves leaning down to your height and kissing your cheek. Especially when your asleep on the couch or in your bed.
Cas-
Forehead. He doesn't fully understand kissing, but he loves how you lightly blush from it and smile up at him shyly.
Gabriel-
Your wrists. He trys really heard to be sweet to you, and he kisses your wrists to let you know that you matter.