That is really a difficult question for me. There are so many different songs I could at any given time say would be my favorite to chill to when in a state of herbal relaxation.
 
If no one minds I will offer more than one suggestion.
 
"Low Spark Of High Heeled Boys" – Traffic
"Kashmir" – Led Zeppelin
"Day After Day," "Come and Get It" – Badfinger
"Whiter Shade of Pale" – Procol Harum
"FM," "Deacon Blues." Hey Nineteen," Do It Again," "My Old School" – Steely Dan
"Valerie," "While You See a Chance" – Steve Winwood
"Damn Good" – "Diamond" David Lee Roth
 
Those would be a few I could at any given time say might by my favorite since the question was about a favorite song to chill to and not favorite song to get into or that pumps you up or that really rocks. I would have totally different songs listed if that had been the case.