Preferably not TOO mainstream.
Here’s mine:
Aisle 10 by Scapegoat Wax (Album is Okeeblow)
I DEFY you to listen to this song twice and ever get it out of your head again.
“Everything Hits at Once” by Spoon
Constantly going through my head.
Can I post two?
Thanks.
“The Fix is In” by OK Go. That’s in my head when the Spoon song isn’t. Sometimes it’s a medly.
I can never get this one out of my head when I hear it:
“Antidote” by Pain
I don’t know if that link works, so if not go here and click on the song file.
Italian Leather Sofa by Cake.
Mr. Bad Example by Warren Zevon
Dreaming of You by The Coral
Gosh, JETGIRL, the lyrics are just so… so… so heartwarming. I’m getting all teary-eyed. Really I am. 
And from this day forth, I’ll think of you every time I hear it!