ABBA

Sensitive writers of genius who emerged out of the German/Swedish schlager world of Hit-Machen, and from Swedish folk... prime musical motor Benny Andersson learnt accordian at 6 from his songwriter grandfather.
The Earlier Years
"Waterloo"
The song to win the Eurovision contest in 1974--this gave them world exposure.
Here is the German version:
"So Long"
A rocker with a dramatic change to the middle section.
"Honey Honey"
Mimed version of the early hit on regional German TV: the host looks like Dracula and calls them "the Abbas". The tune has a great middle section, and is excellently recorded.
"Mamma Mia"
The original clip. Hard to "resist ya".
"SOS"
This is a unique clip of this song, as it is sung live. The backing is pre-recorded. The Who's Pete Townshend once told Benny in New York that "SOS" was the "best [modern popular] song ever written." See the Fender amp behind the girls...
"Dancing Queen"
It was inspired George MacCrae's hit of 1974, "Rock Me Baby".

Live in Germany, 1979
Post "Dancing Queen": Less Stig, more Bjorn
From 1977, Abba wrote a different, slighter darker music. They were more musical theatre or (especially later) disco, but the melodies were still there. The end of both couples' relationships sparked great tunes.
Before this, their publisher, the song-writer Stig Anderson, has written many of their song titles, sometimes the full lyrics--he thought of the title for "Waterloo" and also wrote the words, he wrote the title of "Dancing Queen", of "It's The Name Of The Game", and so on. Bjorn now wrote the lyrics more or less completely, and personal elements came into play.
The final four albums were "The Album" (1977), "Voulez-Vous" (1979), "Super Trouper" (1980--it is thought to be their best) and "The Visitors" (1981). The last was recorded with new digital equipment which seems to have sunk Benny and Bjorn's will to write after that! The biography by Carl Magnus Palm says the sound of the recorder cut off the decay of the voice from a note... Why bother? They should have used the top of the line analog equipment that their engineer had just fully mastered. Digital gear is now better, of course.
Nevertheless, it is a dark album, and dark can work (just ask Goths). Billboard thought it was Abba's "first true masterpiece".
Some later highlights are:
"Thankyou For The Music" (from "The Album")
A good live dub film--Abba write their first overtly "musical theatre" song
A live in Japan (TV) version, with orchestra
Comedy version on BBT TV with Benny at the piano.
"Chiquitita" (English version from "Voulez-Vous")
Abba recorded an entire album of their hits in Spanish on request--that is to say, the girls overdubbed the lyrics while the guys were elsewhere writing new songs. This is the song that inspired the venture.
"I Have A Dream" (from "Voulez-Vous")
A live clip. Bjorn strums a twelve string guitar for that extra chime. An accordian is somewhere--the instrument was Benny's first as a child.
"The Winner Takes It All" (from "Super Trouper")
Abba are said to believe the song is their masterpiece. Anna and Bjorn had just broken up and it is rather obvious...
"When All Is Said And Done" (from "The Visitors")
A demo version:
The released version:
"Slipping Through My Fingers" (from "The Visitors")
Bjorn realized he wouldn't be seeing his daughter go to school every anymore...
"Like An Angel Passing Through My Room" (from "The Visitors")
Madonna heard this as a good example to follow
Like An Angel Passing Through My Room
Maybe this last album was simply Abba's take on the seasonal shift into the Nordic winter. They had had the spring and the summer, and maybe it was now time to take a holiday.