If forced to choose it would be something like Robin Williamson - October Song. I've known this for nearly 40 years and it just keeps getting more and more relevant. Seems almost impossible it was written by a 16 year old.
Well I do have a favorite song of all time and my taste in music will never change and it will always be my favorite song of all time. I don't really even listen to the radio anymore unless I'm in the car because I very nearly dislike all of the new songs that they play anymore and don't love any!! The one's I dislike the most are all of the new Christina Agulara songs. Basically I will love all of the songs that I loved listening to the most while growing up forever. (like a lot of parents who still love all of the beatles songs) I shall put up my playlist on the internet for all to see some time. Then I will tell you all what the most dearest song to me is.
Right now I'm listening to Chevelle - Send The Pain Below and I also like Closure. As for an all time favorite, I can't pick just one, because I like so many different types of music. From rap to rock to clasical, I'll listen to anything.