Music is all about melodies and harmonies to me. Probably the best Pop songs were written between the 60s and 80s. So here's my personal opinion:
1961 - Ben E. King - Stand By Me
1965 - Wanda Jackson - Santa Domingo
1966 - Beach Boys - Sloop John B.
1966 - Chris Farlowe - Out of Time
1967 - Bee Gees - Massachusetts
1967 - Engelbert Humperdinck - Release Me
1968 - The Beatles - Hey Jude
1970 - Deep Purple - Child in Time
1972 - Neil Diamond - Song Sung Blue
1972 - Cats - One Way Wind
1973 - Demis Roussos - My Friend the Wind
1974 - Them - It's All Over Now Baby Blue
1974 - The Rubettes - Sugar Baby Love
1975 - The Carpenters - Please Mr. Postman
1976 - John Miles - Music
1977 - Bellamy Brothers - Crossfire
1977 - Boston - More Than a Feeling
1978 - Bonnie Tyler - It's a Heartache
1980 - REO Speedwagon - Take It On the Run [haven't known this awesome track for a long time!]
1980 - ABBA - I Have a Dream
1981 - Vangelis - Chariots of Fire
1982 - James Last - Biscaya
1984 - Alan Parson's Project - Don't Answer Me
1984 - Fiction Factory - Feels Like Heaven
1984 - Nino De Angelo - Jenseits von Eden
1984 - Alphaville - Forever Young
1985 - Dire Straits - Money For Nothing
1985 - Sandra - Maria Magdalena
1985 - Elton John - Nikita
1985 - Foreigner - I Want To Know What Love Is
1985 - Jennifer Rush - Power of Love
1985 - Peter Maffay - Sonne in der Nacht
1986 - Feargal Sharkey - A Good Heart
1986 - Orchestral Manoeuvres in the Dark - (Forever) Live And Die
1986 - Münchner Freiheit - Ohne Dich
1987 - Kitaro - The Light of the Spirit [rather New Age than Pop]
1987 - Mental As Anything - Live It Up
1987 - Jefferson Starship - Nothing's Gonna Stop Us Now
1987 - Dominoe - Here I Am
1988 - Roxette - The Look
1988 - A-ha - Stay On These Roads
1988 - Whitney Houston - One Moment in Time
1989 - Tina Turner - Simply the Best
I don't know how open minded you are, but I put some german songs as well on this list.
Overall I'd say that the 60's were the most innocent but suffered from bad equipment, the 70's had many great melodies and better sound and the 80's had the best production value ever (peak of analog technique and arrangement). With digital engineering and CDs the music started to focus on beats rather than good song writing. Nevertheless you had a very large band width of music in the 90's charts. Starting with this century, Pop music sounds more or less the same without setting new trends and the look of a performer is probably more important than the music itself.
Actually I thought that "Music" by John Miles is considered as the best Pop song ever written (which I could understand), but according to Rolling Stone's 500 Greatest Songs of All Time it's "Like a Rolling Stone" by Bob Dylan (which I can't understand). In the end it's always a personal opinion, I guess. As always with art, there's no right or wrong.
