Until it’s release as a single in 1983 I would have said TYFTM. I reckon it would have to be either The Way Old Friends Do(quite a few covers) or Slipping Through My Fingers given the success of the first Mamma Mia and DVD’s sold. Know a lot of my colleagues and their daughters liked that one in particular.
I’d still count Thank You For The Music. It was a posthumous single and I wouldn’t count a six-year-old song as a new release. Plus its appearance on Gold (at the expense of Summer Night City) sticks out like a sore thumb. I’d say it’s definitely the most famous non-proper single.
Happy New Year would be second thanks to its seasonal use. It was originally only released as a single in Portugal and a couple of other countries but as it wasn’t a major release I wouldn’t count it as a true single, therefore it’s ABBA’s second most well known non-proper single.
I’d like to add Angeleyes to this category but sadly it’s not nearly famous enough. It counts as a UK/Ireland/US/Canada single but that isn’t really sufficient. It lags a long way behind the first two, regrettably.
The non-singles from the first Mamma Mia! are now famous, but not in their ABBA versions, so I’d exclude those.