Ridley Scott is perhaps not quite as script-centric as Martin Scorsese or able to make a multi-faceted movie like Spielberg or visual effects heavy as George Lucas. However had, Ridley Scott has by far the best visual eye and is also able to draw like a professional designer, which the others can't. I think the best example for this can be seen in both Alien (first one) and Gladiator. In Gladiator he was much more professional from A to Z, but I think the first Alien is even more impressive if you look at it. The camera work is brilliant (Martin also has great camera work in Taxi Driver, but in Alien the effect is IMO much more profound) and the work around for visual problems (depicting the alien without it looking like garbage) was really epic. It is not easily possible to rank directors, but from a visual point of view, Ridley is probably the best individual (visual) artist as such (note: I refer to his abilities, not to the final product, which may not require as much individual artistic prowess, in other words, Martin may not be able to draw something with a pencil, but the end result movie has strong visual effects - see the desert scene in Casino or the "Do I amuse you?" scene in Goodfellas).