Doesn't have to be new or even recent, can be something you caught on the radio that you've never heard before or not heard in a while.
Just tell me your favourite song you've heard this week.
For me it's Sweet Disposition by Temper Trap.
[YOUTUBE]GYCCg-TlPLg[/YOUTUBE]
Don't remember ever listening to this song before it came on the radio this morning but as soon as it came on I knew it well.
I guess it's one of those 4am Friday night songs that I've fell off a table in mojo dancing too.