I can't really pick only ONE, but When I was in my teens my mom thought that "Unwell" by Matchbox20 and Simple Plan's "Welcome to My Life" fit me really well.
Now I would have to say I really like Kelly Clarkson's "My Life Would Suck Without You" is the Perfect one for my BF and I's relationship and Stacie Orrico's "(There's Gotta Be) More to Life" fits me pretty well (see? I graduated from the pretty depressing teenage years songs!

) .