I don’t know what other people would consider their most well-composed song but I have always really loved The Day Before You Came, Eagle, Happy New Year, Our Last Summer….I think my list could go on.
I don’t know if critics agree, but I like “Does Your Mother Know” because, lyrically, it discreetly addresses a questionable situation. Also, the ladies harmonize beautifully, and it’s nice to hear the guys sing lead (since they don’t do it often).
I don’t know about “composed” per se, but I think their best prodiction is ‘SOS’
Other than that one I think probably, Mamma Mia or Dancing Queen; I also used to get a great kick out of ‘Nina, Pretty Ballerina’