One in particular I should mention because it is the only really good song on the album which is "Taking Some Time On" from the first Barclay James Harvest album from 1970. While I think it may be a bit of a harsh statement, there are many that think that was there only good song. I do have to admit the album is a bit of a let down after the first track.
Some more contenders for first best track:
Faust: "It's A Rainy Day, Sunshine Girl", Faust So Far (1972)
John Cale: "A Child's Christmas In Wales", Paris 1919 (1973)
Can: "Future Days", Future Days (1973)
The Who: "Baba O'Riley", Who's Next (1971)
Kinks: "Victoria", Arthur and the Rise and Fall of the British Empire(1969)
The Move: "Hello Suzie", Shazam (1970)
The Mothers Of Invention: "Hungry Freaks, Daddy", Freak Out (1966)
Edgar Broughton Band: "Evening Over Rooftops", Edgar Broughton Band (1971)
The Rolling Stones: "Sympathy For The Devil", Beggar's Banquet (1968)
Of course while on a different subject the worst first track ever on an album was the "song" titled "Procession" off of the Moody Blues Every Good Boy Deserves Favor. The song "The Story In Your Eyes" that immediately follows it is a great track, but that opening segment is a bit of a stinker (and overstays it's welcome).