I had "Does Your Mother Know" stuck in my head for the longest time but I think my favorite could easily be Waterloo or Mamma Mia. Hard to choose, I love most of them.
There's so many that I like. Does your mother know, and when I kissed the teacher were my faves as a kid. I also loved Tiger and my mom recorded me singing it.
I guess as an adult it would be Knowing me knowing you, and the winner takes it all. ( not sure if that's the exact name of it).
I like Slipping Through my Fingers, cause it's really applicable to me and my mum recently since I went off to Uni xD But in general all their songs are so fab.