Donna. Sam. "When All Is Said And Done." It's Meryl Streep and Pierce Brosnan, and the results are exactly what you'd expect from actors of that caliber.
The fact that not a single one of the guys would've been a bad father. They all clearly care about her despite only having just met her, and when they all (separately) work out that they may be her father, all three promise to give her away at her wedding. One gets the distinct sense that, had they known they were possibly her father when she was a child, all three would've gladly stepped up to the plate to be there for her. This bit during Voulez-Vouz is especially sweet (even if Sophie begins to freak out a bit).
Harry: Oh, my God — I've got it! Talk about slow on the uptake! I'm your dad! Sophie: Harry... Harry: No, the penny's dropped now — good and proper — that's why you sent me the invitation. You want your old dad here to walk you down the aisle. Well, I won't let you down! I'll be there!
Donna told her friends - one of whom seems to be the kind of person that would blab at the first chance - that she doesn't know who the father of her child is. They don't turn on her or judge her. They stand by her.
Donna and her friends must be in their mid-fifties. They still get along like a house on fire. How many people can say that?
Related to the above, Donna and her friends utterly freaking out when they reunite at the start of the film. All three of them shriek with joy before running towards each other on the dock, jumping on each other in a Group Hug the first chance they get. The underlying message being, "No matter how grown-up you get, you'll never stop being thrilled to see your best friends."
It's clear that no one judges Donna for having slept with three separate men in such a short period of time that any of them could realistically be Sophie's dad — not Sophie, not her friends, not the men themselves. The lack of Slut-Shaming is very refreshing, and Donna was obviously a very good mom to Sophie, dad or no dad.
Harry hugging Sam after Donna accepts his proposal.