Dancing Queen was their only number one hit in the US, and it was actually inspired by US disco; "Andersson and Ulvaeus have cited
George McCrae's "
Rock Your Baby" as a source of inspiration for the style of the song." (quote from Wikipedia).
I think it is a very good song (particularly the composition and the groove, it's very groovy), but it's not their defining song for me. Actually I can't choose one defining song, since they did so many styles of music. I would have to make a list of five or ten songs to display their versatility in music.
By the way, I saw another enjoyable video recently:
RARE ABBA INTERVIEW: Band members on songs, Mamma Mia and world tour (7NEWS Spotlight)