It was of course sung by the late Harry Nilsson,but you probably DIDN'T know this.Nilsson adapted it from his 1969 song "Girlfriend".If you've ever heard Nilsson's song "Girlfriend",AND the theme,you'll know what I mean.

hi rezny,
actually, since we did the pilot in sept. of 1968, best friend came first. harry wrote it in the studio on the spot for jimmy komack. he recorded girlfriend shortly after.
when i got harry in the studio, to re record best friend, he couldn't remember the chords. we had to call my mom, who was living in portugal at the time, just to get them right.
he didn't even remember the words. it was a classic, but kind of sad, moment. it was also the last time harry ever recorded anything. i'm proud to have been part of that.

thanks for posting,
brandon