How much of Kraftwerk stuff can be considered synth pop? I am assuming most of it is not taken into account, correct me if I am wrong. So I would go with Depeche Mode as #1 and either New Order or The Human League as #2.
It's true that a good portion of Kraftwerk's discography (their early period) wouldn't count here, since they started out as an experimental/progressive electronic group. I'd say it wasn't until Radioactivity
(1975) that they incorporated elements of synth pop. Trans-Europe Express
(1977) followed in a "poppier" vein, and by the time they released The Man-Machine
(1978), they were a full-fledged synth pop act, and arguably the first truly significant one.
I think this string of seminal albums (plus 1981's Computer World
) is enough to merit Kraftwerk the top spot; after all, these releases influenced every synth pop act to follow.