Ok, I never have just one favorite, and it changes all the time anyway,
here are some great ones:
Thin Lizzy: sorry, not one song, the 2 clips are about 45 minutes, but ever second is magic!! a lot of Thunder and Lightning stuff, which is clearly their heaviest work. I love the songs though, this is the one, Holy War, Cold Sweat, Please don't go... doesn't get better than that!
Even though he is probably very much drugged up, he still sings ever word from the bottom of his heart..
also, Scott Gorham and John Sykes is a great duo!!
http://www.youtube.com/watch?v=Cqiwlioiouo
http://www.youtube.com/watch?v=pfA7VZHRKHA