Actually, he was defeated in campaign 6...

Well, that's true, but they didn't kill him. Since the Overlord is immortal, and Relag, Findan, and Zehir will definitely die sometime, either by being killed (like Godric) or from old age, while the Overlord will continue to live long after them.
P.S. 20 years ago, the Sylvan alliance (Alexei, Alaron, and Sirius) also defeated the Overlord, and what happened to them? They were all killed. Heroes come and go, but the Overlord is eternal:confused:
P.P.S. I personally like Argrael-Relag more (probably because of my personal sympathy for the Elves), but Findan is also a great hero, so it's hard to decide.