I wouldn't call most of them "synthpop", that's what I'd label Style Council and Wang Chung. Certainly Kraftwerk and Suicide weren't "synthpop" And, I can't keep 'em to the 70's, because if you want the earliest, you'll need to go back to the 60's.

But OK, call it what you will (I'll call it electronic), I'll be glad to plug some of the earliest good electronic albums, which have given me ages of pleasure:

Terry Riley: A Rainbow In Curved Air (1967) (this avant-garde electronic melange of looping keyboards isn't rock, but it influenced a lot of rock groups of the 70's --inspiring the name of one, Curved Air-- and is a powerful tour de force that will knock you off your feet. I never say "trust me" about anything, but this time, I gotta say "trust me")

The United States of America: The United States of America (1968) (synthesizer, ring modulator, bass, drums, female vocal, organ, but no electric guitars; an alternate psychedelia to the Dead and Airplane; a purely electronic psychedelic album and colorful and intelligent relic of its day) Best songs: "The American Metaphysical Circus" "Hard Coming Down", "I Wouldn't Leave My Wooden Wife for You, Sugar", "The Garden of Earthly Delights"

Other good 60's electronic albums: Fifty Foot Hose - Fifty Foot Hose (1968), Silver Apples - Silver Apples (1968) and Contact (1969) Bruce Haak - Electric Lucifer (1970)

Those are just some of the "famous" ones. There are more mindwarping electronic (or "synthpop") albums from the 60's as well.