Nebulon, I believe was the Defenders Magneto/ Dr. Doom/ Kang. But they also had the Headman. The Emissaries of Evil were supposed to be their "Masters of Evil". And, as I've said before, the Six Fingered Hand is, for me, the high point for the series, though they ended up just being a bunch of minor demons (however, I'm sure the great name could be used again, but with some major enemies of the individual DEFENDERS). Mostly the Defenders used to face one shot mystical menaces or some big adversary of the individual Defenders or of the greater MU. Even Professor Power and the Secret Empire, the New Defenders first major challange, had their start elsewhere (Marvel Team-Up and Captain America). But they did give us Manslaughter.
As for my favorite Defenders stories, The SixFingered Hand. The Guardians of the Galaxy saga. Nebulon and the Squadron Sinister. Gerber's HUGE Headman saga. The Scorpio / LMD Zodiac Cartel. The Overmind / Squadron Supreme. The Professor Power / Secret Empire!
Peace