Each author has his/her own writing style so it's quite hard to say who is better. I admire many authors and Jack London is my most favourite author. I specially like his style, a bit wild, a bit bitter, a bit cold...All senses are combined in his novels. The call of the wild gives me the greatest impression.
Personally, my favourite author is Jean-Paul Sartre (a french author who got a Nobel prize, but who refused it... do'nt know if anyone there knows him )
But I have to admit that Isaac Asimov, in science-fiction, is excellent too !