Asking me to pick one favorite song is like asking me to pick my favorite band. It all depends on what mood I'm in that day. I listen to a very wide variety of music and absolutely love a lot of bands/artists/songs/albums.
When pushed to name my favorite band I always say Lamb Of God. I also can't bring myself to pick one song of theirs that is my favorite. However my favorite album is Ashes of the Wake. Start to finish one of the best American Metal albums period. I often listen to it on repeat as loudly as possible. :D
http://youtu.be/h2WVO1qr5tA
I also absolutely love Seether. And I'd have to say if I have one favorite song Six Gun Quota is it for me. It's my ringtone on my phone. I'm seriously considering getting the entire chorus tattooed on me.
I've burned down every bridge that I've found
Now I limit myself to a six gun quota
I've played down every feelin' I've felt
And I bottled them up 'til the well ran over
http://youtu.be/yqcZ7zXHmTw
It just speaks to me.
I could also talk about numerous other bands and what songs I love but those are my top 2. :)