I'm going to go with Big Brother from 1984 (only read it a few months ago and didn't realise it was popular) because he is truly immortal and he exists in a realistic world. All the others exist in worlds that contain forces that are beyond them. For example Sauron can never be more powerful than magic itself and the emperor isn't more powerful than the force. Big Brother is an idea and in the immortal words of V (V for Vendetta) "ideas are bullet-proof".