The Procol Harum song, which spent six weeks at No 1 during the Summer of Love in 1967, fought off tough competition to claim this title.
Lead singer Gary Brooker said of the honour: "It isn't something that I could have imagined when I wrote the song and then made that legendary recording with Procol Harum all them years ago. Every musician and singer hopes to reach out and communicate with the audience so it means a great deal that the record has such an indefinable popularity and lasting appeal."