I've got a difficult one (at least unable to make a choice myself):
I've Been Waiting For You or So Long?
I've Been Waiting for You
Not your typical ABBA song; acoustic and synth-laden, mid-tempo, and it's in A major. So Long seems like a homage to 50s rock and roll and it has a bit of a "I got you, now you're gonna not see me again!" biting kind of attitude.
I've got a "this or that" question; which drummer would you pick if you were an engineer for ABBA's album? Roger Palm or Ola Brunkert?