As I don't like classifying music genres, I'll just stick to artists where synths form an important part of their sound. I wouldn't really classify most of this as electro or synth-pop. I tried to stick to the most important, but even then, it's still a long list. I just couldn't skip any of these bands.
Aphex Twin
Bisk
Cabaret Voltaire
C-Tec
Current 93
Dead Can Dance
Depeche Mode
Einstürzende Neubauten
Fad Gadget
Front 242
Front Line Assembly
Gary Numan
Herbie Hancock
Hula
Kraftwerk
Male Or Female
Meat Beat Manifesto
Mentallo & The Fixer
Nitzer Ebb
Nouvelles Lectures Cosmopolites
Nurse With Wound
Orbital
Skinny Puppy
Test Dept.
The Black Dog
The Legendary Pink Dots
The Shamen (not all good stuff though)
The Tear Garden
Trisomie 21
Tuxedomoon