There is simply no way for me to provide a definitive answer - I like such a prolific and diverse variety of music that even if I could actually compare them all together somehow to find my "absolute #1 favorite song of all times!", there are just too many possible variables for me to even try. We're talking about a shortlist of thousands here.
So in lieu of typing up an answer that amounts to "I refuse to answer", I'll just tell you what my favorite song at this particular moment is: "Blue and Evil", from Joe Bonamassa's album Black Rock. The "why" should really be self evident after you listen to it - Bonamassa is a rising star/chart-topper in the Blues scene for a damn good reason.
By the time you actually read this of course it is quite probable that I'll have a new "favorite song (of that particular moment)", but this is the one that is running through my head right now and has been all bloody day, and I couldn't be happier because it's awesome.