He might be in the Sci Fi category, i am sure there were great directors in the past as well , there is no definite answer to such claim. Every genre has its own great directors . George Lucas is good but he is far from being the best director in movie history.
George is a great idea man, but as a writer-writing specific scenes and
dialogue-he's weak. He's also a weak director. That's why even great
actors like Neeson, Jackson and MacGregor seemed to give such flat
performances in the prequels. I love the guy, he gave us the greatest
fantasy film series ever.
The prequels are garbage. The original 3 were great, but the prequels destroyed the question of him being the greatest director ever. Steven Spielberg, Stanley Kubrick, even Quentin Tarantino are all tremendously better than George Lucas, who each have repeatedly directed amazing movies, unlike Lucas who merely directed the original Star Wars.
George Lucas is definitely one of the best directors in movie history, but I don't know if you can say he is the best. Star Wars was an incredible script, but we're talking about movie directing and not just script writing. There are many good directors and he is one of them.
George Lucas may be the best director to some people. This is not an objective question—there's no such thing as "best" for a subjective notion like this one. I, for one, do not care about Star Wars. To me, there's tons of other directors whose work is more important, like Orson Welles, Stanley Kubrick, etc.