Don't know about most inspirational song EVER, but "Drops of Jupiter" by Train really helped me through some tough stuff recently.
Other songs that've helped:
"When You Were Young" - The Killers
"How to Save a Life" - The Fray
"Famous Last Words" - My Chemical Romance
"Stay With You" - Goo Goo Dolls
Don't know if any of them are "inspirational" in the traditional sense, but yknow, in their own ways.
There's a TON of songs I use to inspire me to write my fanfiction, but there's way too many of those to list here.